ci: enforce flake.lock correctness

Author: gwennlbh

.github/workflows/nix.yml

@@ -9,3 +9,5 @@ jobs:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v30
       - run: nix build -L
+      - name: Check if there are changes
+        uses: NathanielHill/[email protected]