Marks Mac_arm64 build_tests_2_4 to be unflaky (#142115)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac_arm64 build_tests_2_4"
}
-->
The test has been passing for [50 consecutive runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Mac_arm64%20build_tests_2_4%22).
This test can be marked as unflaky.

.ci.yaml

@@ -3251,7 +3251,6 @@ targets:
 
   - name: Mac_arm64 build_tests_2_4
     recipe: flutter/flutter_drone
-    bringup: true
     timeout: 60
     properties:
       add_recipes_cq: "true"