Marks Windows_arm64 flutter_tool_startup__windows to be unflaky (#137229)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows_arm64 flutter_tool_startup__windows"
}
-->
The issue https://github.com/flutter/flutter/issues/134083 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_arm64%20flutter_tool_startup__windows%22).
This test can be marked as unflaky.

.ci.yaml

@@ -5533,7 +5533,6 @@ targets:
 
   - name: Windows_arm64 flutter_tool_startup__windows
     recipe: devicelab/devicelab_drone
-    bringup: true # https://github.com/flutter/flutter/issues/134083
     presubmit: false
     timeout: 60
     properties: