Console Log. GHC 8.10, cabal 3.4

console-log

~/tmp/du$ cabal install dunai
Resolving dependencies...
Build profile: -w ghc-8.10.4 -O1
In order, the following will be built (use -v for more details):
 - base-orphans-0.8.4 (lib) (requires build)
 - primitive-0.7.1.0 (lib) (requires build)
 - simple-affine-space-0.1.1 (lib) (requires build)
 - splitmix-0.1.0.3 (lib) (requires build)
 - transformers-compat-0.6.6 (lib) (requires build)
 - random-1.2.0 (lib) (requires build)
 - transformers-base-0.4.5.2 (lib) (requires build)
 - MonadRandom-0.5.3 (lib) (requires build)
 - dunai-0.7.0 (lib) (requires download & build)
Downloading  dunai-0.7.0
Starting     base-orphans-0.8.4 (lib)
Starting     simple-affine-space-0.1.1 (lib)
Starting     primitive-0.7.1.0 (lib)
Starting     splitmix-0.1.0.3 (lib)
Downloaded   dunai-0.7.0
Building     base-orphans-0.8.4 (lib)
Building     splitmix-0.1.0.3 (lib)
Building     simple-affine-space-0.1.1 (lib)
Building     primitive-0.7.1.0 (lib)
Installing   splitmix-0.1.0.3 (lib)
Installing   simple-affine-space-0.1.1 (lib)
Completed    splitmix-0.1.0.3 (lib)
Starting     transformers-compat-0.6.6 (lib)
Completed    simple-affine-space-0.1.1 (lib)
Starting     random-1.2.0 (lib)
Building     transformers-compat-0.6.6 (lib)
Building     random-1.2.0 (lib)
Installing   base-orphans-0.8.4 (lib)
Completed    base-orphans-0.8.4 (lib)
Installing   transformers-compat-0.6.6 (lib)
Completed    transformers-compat-0.6.6 (lib)
Starting     transformers-base-0.4.5.2 (lib)
Building     transformers-base-0.4.5.2 (lib)
Installing   transformers-base-0.4.5.2 (lib)
Completed    transformers-base-0.4.5.2 (lib)
Installing   primitive-0.7.1.0 (lib)
Completed    primitive-0.7.1.0 (lib)
Installing   random-1.2.0 (lib)
Completed    random-1.2.0 (lib)
Starting     MonadRandom-0.5.3 (lib)
Building     MonadRandom-0.5.3 (lib)
Installing   MonadRandom-0.5.3 (lib)
Completed    MonadRandom-0.5.3 (lib)
Starting     dunai-0.7.0 (lib)
Building     dunai-0.7.0 (lib)
Installing   dunai-0.7.0 (lib)
Completed    dunai-0.7.0 (lib)
Warning: installdir is not defined. Set it in your cabal config file or use
--installdir=<path>. Using default installdir: "/home/user/.cabal/bin"
Warning:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: Installation might not be completed as desired! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Without flags, the command "cabal install" doesn't expose libraries in a
usable manner. You might have wanted to run "cabal install --lib dunai".
$ cabal repl 
Resolving dependencies...
Build profile: -w ghc-8.10.4 -O1
In order, the following will be built (use -v for more details):
 - fake-package-0 (lib) (first run)
Configuring library for fake-package-0..
Preprocessing library for fake-package-0..
Warning: No exposed modules
GHCi, version 8.10.4: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /tmp/cabal-repl.-24101/setcwd.ghci
Prelude> import Data.MonadicStreamFunction

<no location info>: error:
    Could not load module ‘Data.MonadicStreamFunction’
    It is a member of the hidden package ‘dunai-0.7.0’.
    Perhaps you need to add ‘dunai’ to the build-depends in your .cabal file.
Prelude> 
Leaving GHCi.
$ cabal install --lib dunai
Resolving dependencies...
Up to date
$ cabal repl 
Resolving dependencies...
Build profile: -w ghc-8.10.4 -O1
In order, the following will be built (use -v for more details):
 - fake-package-0 (lib) (first run)
Configuring library for fake-package-0..
Preprocessing library for fake-package-0..
Warning: No exposed modules
GHCi, version 8.10.4: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /tmp/cabal-repl.-24280/setcwd.ghci
Prelude> import Data.MonadicStreamFunction

<no location info>: error:
    Could not load module ‘Data.MonadicStreamFunction’
    It is a member of the hidden package ‘dunai-0.7.0’.
    Perhaps you need to add ‘dunai’ to the build-depends in your .cabal file.
Prelude> 
Leaving GHCi.

I guess so, at that point maybe it worths to create a new whole global config with cabal user-config init --config-file path/to/file and use it with --config-file
I think paths will be relative to that config file.

I think paths will be relative to that config file.

I'd be surprised if that were the case. It it were, that could potentially simplify things a lot.

if they don't, we could try to set cabal_dir env var before the command

our guess was right, user-config init uses $CABAL_DIR as base dir for config values, so changing it is needed, until it includes a new option --cabal-dir (as commented here: haskell/cabal#7455 (comment))

so

D:\ws\haskell\issues\try-dunai2>set CABAL_DIR=D:\ws\haskell\issues\try-dunai2

D:\ws\haskell\issues\try-dunai2>cabal user-config init
Writing default configuration to D:\ws\haskell\issues\try-dunai2\config
D:\ws\haskell\issues\try-dunai2>type .\config
.....
remote-repo-cache: D:\ws\haskell\issues\try-dunai2\packages
-- logs-dir: D:\ws\haskell\issues\try-dunai2\logs
world-file: D:\ws\haskell\issues\try-dunai2\world
....
extra-prog-path: D:\ws\haskell\issues\try-dunai2\bin
...
build-summary: D:\ws\haskell\issues\try-dunai2\logs\build.log
....
installdir: D:\ws\haskell\issues\try-dunai2\bin
.....
etc

After that you can do (without having to set CABAL_DIR in future sessions):

D:\ws\haskell\issues\try-dunai2>cabal --config-file=.\config update
Downloading the latest package list from hackage.haskell.org
Updated package list of hackage.haskell.org to the index-state 2021-09-23T05:14:
06Z
D:\ws\haskell\issues\try-dunai2>dir /B
config
log
packages
store

D:\ws\haskell\issues\try-dunai2>cabal --config-file=.\config repl --build-depends=dunai
Resolving dependencies...
Build profile: -w ghc-8.10.7 -O1
In order, the following will be built (use -v for more details):
 - base-orphans-0.8.5 (lib) (requires download & build)
 - primitive-0.7.2.0 (lib) (requires download & build)
 - simple-affine-space-0.1.1 (lib) (requires download & build)
 - splitmix-0.1.0.3 (lib) (requires download & build)
 - transformers-compat-0.7 (lib) (requires download & build)
 - random-1.2.1 (lib) (requires download & build)
 - transformers-base-0.4.6 (lib) (requires download & build)
 - MonadRandom-0.5.3 (lib) (requires download & build)
 - dunai-0.8.0 (lib) (requires download & build)
 - fake-package-0 (lib) (first run)
 - ............
 - 

will not touch any file in $HOME (i hope)

haskell/cabal#7682

will not touch any file in $HOME (i hope)

Although i a curious about why do you want to isolate all possible dirs, did v2-build store cause you problems? Install dir can be parametrized and the build configured to use binaries produced by previous steps.
As commented the store is immutable and should build and cache new package hashed versions or reuse existing ones in a safe manner, depending on the build config. It sometimes get corrupted due to some bug, but wiping it out usually fixes it
If you want reproducible builds, pining the hackage index and use a cabal freeze file can help.