ci: add update-flake.yml

.github/workflows/update_flake.yml

@@ -0,0 +1,23 @@
+name: flake-update
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 0 * * 0"
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check repository
+        uses: actions/checkout@v4
+        with:
+          ref: "lagrange"
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+        with:
+          pr-title: "Update flake inputs"
+          pr-labels: |
+            dependencies
+            automated