Add flake update cron workflow

.github/workflows/flake-update.yaml

@@ -0,0 +1,26 @@
+name: Periodic flake update
+on:
+  schedule:
+    - cron: '0 0 * * *'
+  workflow_dispatch:
+
+  permissions:
+    contents: write
+
+jobs:
+  flake-update:
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/catthehacker/ubuntu:runner-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v27
+      - name: configure git
+        run: |
+          git config --global user.name 'torjus-bot'
+          git config --global user.email 'torjus-bot@git.t-juice.club'
+      - name: flake update
+        run: nix flake update --commit-lock-file
+      - name: push
+        run: git push
+