Marks Linux_pixel_7pro service_extensions_test to be flaky (flutter#157853)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_pixel_7pro service_extensions_test"
}
-->
Issue link: flutter#157852

Author: fluttergithubbot

.ci.yaml

@@ -3284,6 +3284,7 @@ targets:
       task_name: routing_test
 
   - name: Linux_pixel_7pro service_extensions_test
+    bringup: true # Flaky https://github.com/flutter/flutter/issues/157852
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60