Add flake checker action to CI

.github/workflows/ci.yml

@@ -11,6 +11,10 @@ jobs:
       - uses: actions/checkout@v3
         with:
           fetch-depth: 0
+      - name: Check Nixpkgs input
+        uses: DeterminateSystems/flake-checker-action@v3
+        with:
+          fail-mode: true
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@main
       - name: Shellcheck