ci: run nix flake check instead of nix build

Signed-off-by: Gwenn Le Bihan <[email protected]>

Author: gwennlbh

.github/workflows/nix.yml

@@ -8,6 +8,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v30
-      - run: nix build -L
+      - run: nix flake check -L
       - name: Check if there are changes
         uses: NathanielHill/[email protected]