Merge branch 'nixpkgs-git-revision-fix'

Addresses #17218 in a better way in that it doesn't create a
".git-revision" file on every nixos-rebuild, because we already have
".git" available. Even if we don't nixos-rebuild can't create the
"git-revision" file.

Tested via:

nix-build -E '(import ./nixos/tests/make-test.nix {
  name = "foo";
  machine = {};
  testScript = "startAll; $machine->execute(\"nixos-version >&2\");";
})'

Closes: #17610
Acked-by: @bennofs