github: Reintroduce update-flake-lock.yaml

But with cron disabled.

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

@@ -0,0 +1,24 @@
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  # schedule:
+  #   - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Install Nix
+        uses: cachix/install-nix-action@v17
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@v14
+        with:
+          token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
+          pr-title: "Update flake.lock" # Title of PR to be created
+          pr-labels: |                  # Labels to be set on the PR
+            automated