nix + ssh-agent + git-config

gistfile1.txt

➜ sudo find /etc/nix
/etc/nix
/etc/nix/ssh
/etc/nix/ssh/known_hosts
/etc/nix/ssh/config
/etc/nix/nix.custom.conf
/etc/nix/nix.conf
/etc/nix/gitconfig
➜ sudo cat /etc/nix/ssh/config
Host github.com
    IdentityAgent /etc/systemd/system/nix-daemon-ssh-agent.sock
    StrictHostKeyChecking no
    UserKnownHostsFile /etc/ssh/ssh/known_hosts
➜ sudo cat /etc/nix/gitconfig 
[filter "lfs"]
        process = git-lfs filter-process
        required = true
        clean =...%f
[url "[email protected]:truvity/"]
        insteadOf = https://github.com/truvity/
➜ sudo cat /etc/nix/nix.custom.conf
# Written by https://github.com/DeterminateSystems/nix-installer.
# The contents below are based on options specified at installation time.
extra-sandbox-paths = /etc/systemd/system/nix-daemon-ssh-agent.sock?
extra-sandbox-paths = /etc/gitconfig=/etc/nix/gitconfig
extra-sandbox-paths = /etc/ssh/ssh_config=/etc/nix/ssh/config
builders-use-substitutes = true
➜ sudo cat /etc/systemd/system/nix-daemon-ssh-agent.service 
[Unit]
Description=SSH agent for Nix builds
After=network.target

[Service]
Type=forking
User=root
ExecStart=/usr/local/bin/nix-daemon-ssh-agent
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target
➜ sudo cat /usr/local/bin/nix-daemon-ssh-agent
#!/bin/bash

set -euo pipefail

export USER_SSH_AUTH_SOCK=/run/user/1000/gnupg/S.gpg-agent.ssh
export COPY_SSH_AUTH_SOCK=/etc/systemd/system/nix-daemon-ssh-agent.sock
(umount $COPY_SSH_AUTH_SOCK || true)
touch $COPY_SSH_AUTH_SOCK
/usr/bin/mount --bind $USER_SSH_AUTH_SOCK $COPY_SSH_AUTH_SOCK
chmod 666 $COPY_SSH_AUTH_SOCK
➜ sudo bash -c 'ssh-keyscan -H github.com > /etc/nix/ssh/known_hosts'
➜ sudo chmod 0700 /etc/nix/ssh
➜ sudo chmod 0644 /etc/nix/ssh/known_hosts
➜ sudo chmod 0644 /etc/nix/ssh/config
➜ sudo systemctl enable --now /etc/systemd/system/nix-daemon-ssh-agent.service
➜ sudo systemctl restart nix-daemon.service

all good, except for one thing:
sudo chmod +x /usr/local/bin/nix-daemon-ssh-agent, otherwise you get this error:

nix-daemon-ssh-agent.service: Failed with result 'exit-code'.

@kmaxat-truvity thank you!