ci(gcb): create secrets if necessary (#6316)
ci(gcb): create secrets if necessary (#6316)

* ci(gcb): create secrets if necessary

Our GCB builds expect secrets for the `CODECOV_TOKEN` and the
`LOG_LINKER_PAT`. These exist in our main GCB project, but not in our
personal projects. This PR creates these secrets with empty data if
they don't already exist.

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