test(GCS+gRPC): speed up `GrpcClient` test (#9050)
test(GCS+gRPC): speed up `GrpcClient` test (#9050)

On Windows the default credentials are fairly slow, and we do not need
them for this unit test.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp