Marks Windows plugin_test to be flaky (flutter#148835)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows plugin_test"
}
-->
Issue link: flutter#148834

.ci.yaml

@@ -5455,6 +5455,7 @@ targets:
       - .ci.yaml
 
   - name: Windows plugin_test
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/148834
     recipe: devicelab/devicelab_drone
     timeout: 60
     properties: