gitlab-ci: fix Nix format check

The new version of Nix no longer provides path to the source to the
formatter and thus alehandra uses stdin instead and thus run fails.

The Nix functionality change: NixOS/nix#11438

Author: Cynerd

.gitlab-ci.yml

@@ -54,5 +54,4 @@ nixfmt:
   extends: .nix
   allow_failure: true
   script:
-    - nix fmt
-    - git diff --exit-code
+    - nix fmt -- --check .