[CI] Open PR with updated flake inputs

.github/workflows/update.yaml

@@ -0,0 +1,30 @@
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 15 * * *' # running daily at 00:15 AM
+
+jobs:
+  update-lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - uses: cachix/install-nix-action@v19
+        with:
+          extra_nix_config: |
+            accept-flake-config = true
+      - uses: cachix/cachix-action@v12
+        with:
+          name: aciceri-emacs
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@v16
+        with:
+          pr-title: "Automatic `flake.lock` update"
+          pr-labels: |
+            flake-inputs
+            automatic
+          token: ${{ secrets.PR_UPDATE_FLAKE_TOKEN }} # to open the PR using my GitHub account, needed to trigger the `build` workflow