CI test linux://release:test_cluster_manager is consistently_failing

[aslonnie]

CI test linux://release:test_cluster_manager is consistently_failing. Recent failures:
- https://buildkite.com/ray-project/postmerge/builds/15459#019bc4a9-e25c-4c64-b5e5-f779e766fbd8
- https://buildkite.com/ray-project/postmerge/builds/15459#019bc498-7feb-45e6-974b-0b104668183f

DataCaseName-linux://release:test_cluster_manager-END
Managed by OSS Test Policy

[aslonnie]

This test is now considered as flaky because it has been failing on postmerge for too long. Flaky tests do not run on premerge.