ci: clang-tidy the GCS+gRPC plugin code (#6324)
ci: clang-tidy the GCS+gRPC plugin code (#6324)

We need to enable the GCS+gRPC plugin on the clang-tidy build or the
files for it do not get "tidied".
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp