It would be great if I can specify the bucket name to upload source,
just like --gcs-source-staging-dir option in "gcloud builds submit".

Current behavior, creating "jenkins-tmp_xxxx" causes following problems:

• Requires to assign storage admin role to the service account for Jenkins. I want to restrict that service account with fewer permissions.
• I want to manage my storage in my rule. (like projectname_cloudbuild-source).