chore: turn on waitOnContentHash flag (#4706)

Enable `waitOnContentHash` for the merge queue.

If something goes wrong; revert this PR and wait ~2 minutes.

Author: jtmcdole

app_dart/config.yaml

@@ -4,3 +4,8 @@
 # app_dart/lib/src/service/config.dart
 
 backfillerCommitLimit: 50
+
+contentAwareHashing:
+  # This will cause PRs that enter the merge queue to wait on CAH hashing
+  # to happen before scheduling builds.
+  waitOnContentHash: true