feat: add formatting action (#28)

.github/workflows/format.yml

@@ -0,0 +1,33 @@
+---
+name: Format
+on:
+  push:
+    branches: ["*"]
+  pull_request:
+    branches: ["*"]
+jobs:
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install just
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://just.systems/install.sh | sudo bash -s -- --to /usr/local/bin
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v20
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - name: Install nixfmt-rfc-style
+        run: nix-env -i nixfmt-rfc-style
+
+      - name: Run format command
+        run: just fmt
+
+      - name: Commit changes
+        uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          commit_message: "Auto-format Nix files"
+          file_pattern: "*.nix"