[ci] move //:gcs_placement_group_manager_test to flaky (#41216)

This test has gone bad, blocking postmerge at least 4 times now.

Open issue: #41215

Signed-off-by: can <can@anyscale.com>

ci/ray_ci/core.tests.yml

@@ -1,3 +1,4 @@
 flaky_tests:
   - //python/ray/tests:test_client_library_integration
   - //src/ray/common/test:ray_syncer_test
+  - //:gcs_placement_group_manager_test