Marks Mac platform_channel_sample_test_macos to be flaky (#151884)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac platform_channel_sample_test_macos"
}
-->
Issue link: flutter/flutter#151883

Author: fluttergithubbot

.ci.yaml

@@ -3923,6 +3923,7 @@ targets:
       task_name: platform_view_macos__start_up
 
   - name: Mac platform_channel_sample_test_macos
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/151883
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60