Fix for error during NixOS UEFI installation: "Failed to create EFI Boot variable entry: No such file or directory"

nixos-install.log

# Tested with nixos-minimal-18.03pre130932.cc4677c36ee-x86_64-linux.iso
# Manually mounted /mnt (NixOS root), /mnt/boot (mkfs.ext4 Grub partition), /mnt/boot/efi (mkfs.vfat for EFI boot vars)

[root@nixos:~]# cat /mnt/etc/nixos/hardware-configuration.nix 
# Do not modify this file!  It was generated by ‘nixos-generate-config’
# and may be overwritten by future invocations.  Please make changes
# to /etc/nixos/configuration.nix instead.
{ config, lib, pkgs, ... }:

{
  imports =
    [ <nixpkgs/nixos/modules/installer/scan/not-detected.nix>
    ];

  boot.initrd.availableKernelModules = [ "xhci_pci" "ahci" "nvme" "usb_storage" "sd_mod" "rtsx_pci_sdmmc" ];
  boot.kernelModules = [ "kvm-intel" ];
  boot.extraModulePackages = [ ];

  fileSystems."/" =
    { device = "/dev/disk/by-uuid/8ee6694b-c5fd-408c-a62b-bff3fed8279b";
      fsType = "btrfs";
    };

  fileSystems."/boot" =
    { device = "/dev/disk/by-uuid/b08fc826-a03a-4a55-b58c-c79817c274fa";
      fsType = "ext4";
    };

  fileSystems."/boot/efi" =
    { device = "/dev/disk/by-uuid/DCAD-4D1C";
      fsType = "vfat";
    };

  swapDevices =
    [ { device = "/dev/disk/by-uuid/1e680b2f-a23a-40bc-8143-00c50820b715"; }
    ];

  nix.maxJobs = lib.mkDefault 4;
  powerManagement.cpuFreqGovernor = lib.mkDefault "powersave";
}

[root@nixos:~]# cat /mnt/etc/nixos/configuration.nix | head -n20
# Edit this configuration file to define what should be installed on
# your system.  Help is available in the configuration.nix(5) man page
# and in the NixOS manual (accessible by running ‘nixos-help’).

{ config, pkgs, ... }:

{
  imports =
    [ # Include the results of the hardware scan.
      ./hardware-configuration.nix
    ];

  # Use the systemd-boot EFI boot loader.
  boot.loader.systemd-boot.enable = true;
  boot.loader.efi.canTouchEfiVariables = true;

  # Use /boot/efi as the EFI mount point instead of /boot.
  boot.loader.efi.efiSysMountPoint = "/boot/efi";

  # ...

[root@nixos:~]# nixos-install
building the configuration in /mnt/etc/nixos/configuration.nix...
warning: dumping very large path (> 256 MiB); this may run out of memory
[164 built, 257 copied (1332.4 MiB), 10.6 MiB DL]
copying channel...
installing the boot loader...
setting up /etc...
Initializing machine ID from random generator.
Created "/boot/efi/EFI".
Created "/boot/efi/EFI/systemd".
Created "/boot/efi/EFI/BOOT".
Created "/boot/efi/loader".
Created "/boot/efi/loader/entries".
Copied "/nix/store/7m31df5kk2s0s6b4j0whky06g03ayjns-systemd-237/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/efi/EFI/systemd/systemd-bootx64.efi".
Copied "/nix/store/7m31df5kk2s0s6b4j0whky06g03ayjns-systemd-237/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/efi/EFI/BOOT/BOOTX64.EFI".
Failed to create EFI Boot variable entry: No such file or directory
Traceback (most recent call last):
  File "/nix/store/xg92k6fil20qqb2cssd1513b8189z7p6-systemd-boot-builder.py", line 210, in <module>
    main()
  File "/nix/store/xg92k6fil20qqb2cssd1513b8189z7p6-systemd-boot-builder.py", line 185, in main
    subprocess.check_call(["/nix/store/7m31df5kk2s0s6b4j0whky06g03ayjns-systemd-237/bin/bootctl", "--path=/boot/efi", "install"])
  File "/nix/store/p1bp0kpmxi7nzrla7n7c4waqic0a2myk-python3-3.6.4/lib/python3.6/subprocess.py", line 291, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/nix/store/7m31df5kk2s0s6b4j0whky06g03ayjns-systemd-237/bin/bootctl', '--path=/boot/efi', 'install']' returned non-zero exit status 1.

[root@nixos:~]# mount -t sysfs sysfs /mnt/sys

[root@nixos:~]# mount -t efivarfs efivarfs /mnt/sys/firmware/efi/efivars/

[root@nixos:~]# nixos-install
building the configuration in /mnt/etc/nixos/configuration.nix...
copying channel...
installing the boot loader...
setting up /etc...
Copied "/nix/store/7m31df5kk2s0s6b4j0whky06g03ayjns-systemd-237/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/efi/EFI/systemd/systemd-bootx64.efi".
Copied "/nix/store/7m31df5kk2s0s6b4j0whky06g03ayjns-systemd-237/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/efi/EFI/BOOT/BOOTX64.EFI".
Created EFI boot entry "Linux Boot Manager".
setting up /etc...
setting root password...
...
Created EFI boot entry "Linux Boot Manager".
Installation finished!