ci: add workflow to upgrade flake locks every month

.github/workflows/monthly-upgrade.yml

@@ -0,0 +1,26 @@
+name: Monthly Upgrade
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 28 * *'
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Get current month
+        run: |
+          echo "CURRENT_MONTH=$(date +'%B')" >> $GITHUB_ENV
+          echo "CURRENT_YEAR=$(date +'%Y')" >> $GITHUB_ENV
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+        with:
+          token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
+          pr-title: "Monthly Flake Upgrade (${{ env.CURRENT_MONTH }}, ${{ env.CURRENT_YEAR }})"
+          pr-labels: |
+            dependencies
+            automated