add update lock action

.forgejo/workflows/update-lock.yml

@@ -0,0 +1,31 @@
+name: Update flake.lock
+
+on:
+  workflow_dispatch:
+
+jobs:
+  update_flake_lock:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v14
+
+      - name: Set git config
+        run: |
+          git config --global user.name "ukubot"
+          git config --global user.email "bot@uku.moe"
+
+      - name: Update lockfile
+        run: |
+          export BRANCH="update-lock-$(date -I)"
+          git checkout -b "$BRANCH"
+          nix flake update --commit-lock-file
+          git push origin "$BRANCH"
+
+      - name: Create Pull Request
+        run: |
+          echo EXPLODE