automate updating nix flakes (#35641)

.github/workflows/cron-flake-updater.yml

@@ -0,0 +1,22 @@
+name: cron-flake-updater
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
+
+jobs:
+  nix-flake-update:
+    permissions:
+      contents: write
+      issues: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: DeterminateSystems/determinate-nix-action@v3
+      - uses: DeterminateSystems/update-flake-lock@main
+        with:
+          pr-title: "Update Nix flake"
+          pr-labels: |
+            dependencies