Marks Linux_pixel_7pro service_extensions_test to be unflaky (flutter#176700)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_pixel_7pro service_extensions_test"
}
-->
The issue flutter#174113 has been
closed, and the test has been passing for [50 consecutive
runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Linux_pixel_7pro%20service_extensions_test%22).
This test can be marked as unflaky.

Author: fluttergithubbot

.ci.yaml

@@ -3386,7 +3386,6 @@ targets:
       task_name: routing_test
 
   - name: Linux_pixel_7pro service_extensions_test
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/174113
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60