add default nix formatter and its corresponding Github action

.github/workflows/nix-fmt-checks.yaml

@@ -0,0 +1,17 @@
+name: Nix formatter checks
+
+on:
+  pull_request:
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v18
+
+      - name: Run nix formatter tool
+        run: nix fmt . -- --check

flake.nix

@@ -28,6 +28,8 @@
           ];
         };
       in rec {
+        formatter = pkgs.alejandra;
+
         packages = {
           inherit (pkgs) php php56 php70 php71 php72 php73 php74 php80 php81 php82;
         };