Add auto-update action

.github/workflows/flake-auto-update.yml

@@ -0,0 +1,27 @@
+name: Auto update
+
+on: push
+
+jobs:
+  format-code:
+    runs-on: ubuntu-latest
+
+    permissions:
+      # Give the default GITHUB_TOKEN write permission to commit and push the
+      # added or changed files to the repository.
+      contents: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v25
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+      # Other steps that change files in the repository
+      - run : nix flake update
+
+      # Commit all changed files back to the repository
+      - uses: stefanzweifel/git-auto-commit-action@v5
+        with:
+          commit_message: Auto update