Marks Windows windows_desktop_impeller to be unflaky (flutter#158565)

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

Author: fluttergithubbot

.ci.yaml

@@ -6370,7 +6370,6 @@ targets:
 
   - name: Windows windows_desktop_impeller
     recipe: devicelab/devicelab_drone
-    bringup: true
     presubmit: false
     timeout: 60
     properties: