ci: add update flake workflow

Author: felbinger

.github/workflows/update.yaml

@@ -0,0 +1,45 @@
+name: Update Flake
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "43 3 */10 * *" # run every ten days at 3:43 am
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  flake-update:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+        with:
+          fetch-depth: 0
+
+      - uses: DeterminateSystems/nix-installer-action@c5a866b6ab867e88becbed4467b93592bce69f8a # v21
+
+      - name: Set up Git user
+        run: |
+          git config user.name 'github-actions[bot]'
+          git config user.email 'github-actions[bot]@users.noreply.github.com'
+
+      - name: Create or switch to 'update' branch
+        run: |
+          git branch -f update origin/main
+          git checkout update
+
+      - name: nix flake update
+        run: nix flake update --commit-lock-file
+
+      - name: Push changes
+        run: git push --force-with-lease origin update
+
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@271a8d0340265f705b14b6d32b9829c1cb33d45e # v7.0.8
+        with:
+          title: Update Flake
+          branch: update
+          base: main