add a manually triggered flake update ci job

the job will run `nix flake update` and create a PR with the changes

.github/workflows/flake-update.yml

@@ -0,0 +1,24 @@
+name: Flake update
+
+on: workflow_dispatch
+
+jobs:
+  upgrade:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v24
+    - run: nix flake update
+    - run: nix build --no-update-lock-file .#release-info
+
+    - name: Create Pull Request
+      uses: peter-evans/create-pull-request@v5
+      with:
+        title: "[Automated] update flake"
+        commit-message: "[Automated] update flake"
+        branch: "auto/flake-update"
+        add-paths: flake.lock
+        delete-branch: true
+        labels: |
+            automated pr
+        body-path: result