Marks Mac channels_integration_test to be flaky (#151882)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Mac channels_integration_test"
}
-->
Issue link: flutter/flutter#151881

.ci.yaml

@@ -5000,6 +5000,7 @@ targets:
       - .ci.yaml
 
   - name: Mac channels_integration_test
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/151881
     recipe: devicelab/devicelab_drone
     timeout: 60
     properties: