restore flake-update action

.github/workflows/flake-update.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   createPullRequest:
     runs-on: ubuntu-latest
-    # if: github.ref == 'refs/heads/main' # ensure workflow_dispatch only runs on main
+    if: github.ref == 'refs/heads/main' # ensure workflow_dispatch only runs on main
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
@@ -22,8 +22,6 @@ jobs:
         id: update
         uses: DeterminateSystems/update-flake-lock@main
         with:
-          pr-title: "Update flake.lock ${{ github.ref }}" # Title of PR to be created
-          branch: "update-${{ github.ref }}"
           pr-labels: |                  # Labels to be set on the PR
             dependencies
             automated