Fix CI: install alejandra and deadnix before pre-commit

The alejandra-system and deadnix hooks expect the binaries on PATH.
Install them via nix profile before running pre-commit.

Assisted-by: Claude Code
Signed-off-by: Miroslav Vadkerti <mvadkert@redhat.com>

Author: thrix

.github/workflows/check.yml

@@ -13,6 +13,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v31
+      - run: nix profile install nixpkgs#alejandra nixpkgs#deadnix
       - uses: pre-commit/action@v3.0.1
 
   nix-build: