[labeler] Set sync labels to false to stop removing labels (#128446)

https://github.com/flutter/flutter/issues/128440

The current version of the action has a bug where the sync-labels default value is not read correctly. Explicitly setting to see if that stops the removals.

.github/workflows/labeler.yml

@@ -18,3 +18,5 @@ jobs:
     steps:
     # Source available at https://github.com/actions/labeler/blob/main/README.md
     - uses: actions/labeler@9fcb2c2f5584144ca754f8bfe8c6f81e77753375
+      with:
+        sync-labels: false