chore: add ci job to ensure formatting is correct

.github/workflows/format.yaml

@@ -0,0 +1,24 @@
+name: "format"
+on:
+  pull_request:
+  push:
+    branches: [ "main" ]
+
+jobs:
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v5
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Setup Caching
+        uses: cachix/cachix-action@v16
+        with:
+          name: selfhostblocks
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+      - name: Check Formatting
+        run: |
+          find . -name '*.nix' | nix fmt -- --ci