Add Action to update lockfile

.github/workflows/update.yaml

@@ -0,0 +1,26 @@
+name: update-flake-lock
+on:
+  workflow_dispatch:
+  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@v16
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+      - name: Update flake.lock
+        id: update
+        uses: DeterminateSystems/update-flake-lock@v16
+        with:
+          pr-title: "Update flake.lock"
+      # - uses: peter-evans/enable-pull-request-automerge@v2
+      #   with:
+      #     pull-request-number: ${{ steps.update.outputs.pull-request-number }}
+      #     merge-method: rebase