ci: weekly update flake lock

.github/workflows/update-flake-lock.yml

+name: update-flake-lock
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 1 3 * *'
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - 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