Run lock workflow only in flutter/flutter. (#87601)

This is to prevent trying to run the workflow in forks.

Bug: https://github.com/flutter/flutter/issues/87573

.github/workflows/lock.yaml

@@ -9,6 +9,7 @@ on:
 jobs:
   lock:
     runs-on: ubuntu-latest
+    if: ${{ github.repository == 'flutter/flutter' }}
     steps:
       - uses: dessant/lock-threads@486f7380c15596f92b724e4260e4981c68d6bde6
         with: