github/workflows: add update-flake-lock action to bump deps (#284)

This daily tests if nixpkgs updates breaks our interface.

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

@@ -0,0 +1,25 @@
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '00 17 * * *' # runs daily.
+
+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
+        uses: DeterminateSystems/update-flake-lock@v16
+        with:
+          pr-title: "flake: update the lock" # Title of PR to be created
+          pr-labels: |                  # Labels to be set on the PR
+            dependencies
+            automated
+          token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}