Create update flake workflow

.forgejo/workflows/update.yml

@@ -0,0 +1,13 @@
+on:
+  schedule:
+    - cron: '0 2 * * *'
+jobs:
+    lockfile:
+        runs-on: ubuntu-22.04
+        steps:
+            - name: checkout
+              uses: actions/checkout@v4
+            - name: Install Nix
+              uses: https://github.com/DeterminateSystems/nix-installer-action@v1
+            - name: Update flake.lock
+              uses: https://github.com/DeterminateSystems/update-flake-lock@main