ci: allow --verbose flag to affect output of docker build (#9119)
ci: allow --verbose flag to affect output of docker build (#9119)

`ci/cloudbuild/build.sh --docker --trigger=xxx --verbose` failed
to actually be verbose in the local build within the docker image
because the `--env=VERBOSE_FLAG=${VERBOSE_FLAG:-}` setting was
ignored in the `ci/cloudbuild/build.sh --local` reinvocation.
So, make the `VERBOSE_FLAG` initialization heed an existing value
in the environment.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp