ci: ensure flake is formatted

.github/workflows/nix.yml

@@ -0,0 +1,22 @@
+name: Test Nix flake
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  format:
+    name: check flake formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          show-progress: false
+      - uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - run: nix fmt
+
+      # Check that formatting does not change anything.
+      - run: git diff --exit-code