Add GitHub Actions flow to update flake lock automatically

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/auto-update.yml

@@ -0,0 +1,24 @@
+name: 'Flake.lock: update Nix dependencies'
+
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '37 17 * * 5' # runs weekly on Friday at 17:37
+
+jobs:
+  nix-flake-update:
+    permissions:
+      contents: write
+      id-token: write
+      issues: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/determinate-nix-action@v3
+      - uses: DeterminateSystems/update-flake-lock@main
+        with:
+          pr-title: 'Update Nix flake inputs' # Title of PR to be created
+          pr-labels: | # Labels to be set on the PR
+            dependencies
+            automated