Marks Windows module_custom_host_app_name_test to be unflaky (#135961)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows module_custom_host_app_name_test"
}
-->
The issue flutter/flutter#134644 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:%22Windows%20module_custom_host_app_name_test%22).
This test can be marked as unflaky.

.ci.yaml

@@ -4676,7 +4676,6 @@ targets:
       task_name: hot_mode_dev_cycle_win_target__benchmark
 
   - name: Windows module_custom_host_app_name_test
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/134644
     recipe: devicelab/devicelab_drone
     timeout: 60
     properties: