ci: add workflow to check nix files format

.github/workflows/style.yml

@@ -0,0 +1,16 @@
+name: Check *.nix files format
+on: [push, pull_request]
+
+jobs:
+  check-formatting:
+    name: Check formatting
+    runs-on: ubuntu-latest
+    steps:
+      - name: Code checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Alejandra CLI
+        uses: luisnquin/setup-alejandra@v1.0.0
+
+      - name: Check formatting
+        run: alejandra --check **/*.nix