fix: use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR (#8477)
fix: use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR (#8477)

Co-authored-by: Carlos O'Ryan <9e621187562cef94ec9617928f4eb1eb11d57112@google.com>

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