ci: use release version of Kaniko (#8711)
ci: use release version of Kaniko (#8711)

We use `kaniko:v1.6.y-debug` instead of `kaniko:v1.6.y` to workaround
problems I think do not exist in the v1.8.x series (and higher).
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp