Marks Windows build_tests_3_8 to be unflaky (#157414)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows build_tests_3_8"
}
-->
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%20build_tests_3_8%22).
This test can be marked as unflaky.

Author: fluttergithubbot