ci(gcb): add elapsed time to Ninja builds (#6186)
ci(gcb): add elapsed time to Ninja builds (#6186)

This should help us understand our build performance and possibly
diagnose timeouts like
https://github.com/googleapis/google-cloud-cpp/issues/6184
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp