ci: only clang-tidy headers in the .clang-tidy regexp (#4097)
ci: only clang-tidy headers in the .clang-tidy regexp (#4097)

Many of our headers are not clang-tidy clean, so we need to limit the
regular expression to whatever is on the `.clang-tidy` file.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp