nixos: fix "nixos-rebuild build-vm-with-bootloader" for EFI systems

[bjornfor]

Motivation for this change

nixos-rebuild build-vm-with-bootloader currently fails with the
default NixOS EFI configuration:

  $ cat >configuration.nix <<EOF
  {
    fileSystems."/".device = "/dev/sda1";
    boot.loader.systemd-boot.enable = true;
    boot.loader.efi.canTouchEfiVariables = true;
  }
  EOF

  $ nixos-rebuild build-vm-with-bootloader -I nixos-config=$PWD/configuration.nix -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-20.09.tar.gz
  [...]
  insmod: ERROR: could not insert module /nix/store/1ibmgfr13r8b6xyn4f0wj115819f359c-linux-5.4.83/lib/modules/5.4.83/kernel/fs/efivarfs/efivarfs.ko.xz: No such device
  mount: /sys/firmware/efi/efivars: mount point does not exist.
  [    1.908328] reboot: Power down
  builder for '/nix/store/dx2ycclyknvibrskwmii42sgyalagjxa-nixos-boot-disk.drv' failed with exit code 32
  [...]

Fix it by setting virtualisation.useEFIBoot = true in qemu-vm.nix, when
efi is neeeded.

And remove the now unneeded configuration in
./nixos/tests/systemd-boot.nix, since it's handled globally.

Before:

• release-20.03: successful build, unsuccessful run
• release-20.09 (and master): unsuccessful build

After:

• Successful build and run.

Fixes #107255

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[danielfullmer]

LGTM. This seems like the most reasonable place to put this, given the choices here: #107255 (comment)

[rnhmjoj]

This still fails if systemd-boot is not used. Minimal way to reproduce:

nix-build -E '(import ./nixos {
  configuration = {
    boot.loader.efi.canTouchEfiVariables = true;
  };
}).vmWithBootLoader'

[bjornfor]

@rnhmjoj: Thanks, fixed now.

[rnhmjoj]

Looks good, now.

[bjornfor]

Backported to release-20.09: 9a5a4c7

[vcunat]

I reverted this merge. It was blocking the channel which is very far behind ATM, so I didn't even try understanding the problem. Please test nixosTests.installer.simpleUefiSystemdBoot before merging again.

[bjornfor]

Sorry!

[bjornfor]

v2 of this change is here: #108504