ci: add a new auto-update workflow

.gitea/workflows/auto-update.yaml

@@ -0,0 +1,25 @@
+name: Bump flake.lock
+on:
+  schedule:
+    - cron: "0 6 * * *"
+  workflow_dispatch:
+jobs:
+  build:
+    runs-on: nix-runner
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: cachix/install-nix-action@v20
+      - run: nix flake update
+      - uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          commit_message: Bump flake.lock
+          branch: flake-update
+          commit_options: '--no-verify --signoff'
+          file_pattern: flake.lock
+          commit_user_name: Flake Bot # defaults to "GitHub Actions"
+          #commit_user_email: my-github-actions-bot@example.org # defaults to "actions@github.com"
+          commit_author: Flake Bot <actions@github.com> # defaults to author of the commit that triggered the run
+          skip_dirty_check: false
+          skip_fetch: true