Mark platform_channel_sample_test_macos unflaky (#160989)

This should have been marked unflaky when
https://github.com/flutter/flutter/issues/151883 was closed.

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.ci.yaml

@@ -4332,7 +4332,6 @@ 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