Add CLOUD_BUILD_GCS_LOG_DIR option to read cloud build logs

[katsuyan]

It faild reading the cloud build logs When GAE_EXEC_STRATEGY is cloud_build.

ERROR: (gcloud.container.builds.submit) HTTPError 403: <?xml version='1.0' encoding='UTF-8'?><Error><Code>AccessDenied</Code><Message>Access denied.</Message><Details>circleci@project.iam.gserviceaccount.com does not have storage.objects.get access to PROJECT_ID.cloudbuild-logs.googleusercontent.com/log-xxxxxxxxxxxxxxx.txt.</Details></Error>

I found the following Github issue.

https://github.com/GoogleCloudPlatform/cloud-builders/issues/120

So, I add CLOUD_BUILD_GCS_LOG_DIR option to add --gcs-log-dir option for gcloud command.

Usage

bundle exec rake CLOUD_BUILD_GCS_LOG_DIR="gs://appengine-katsuyan-test/cloudbuild" appengine:exec -- bundle exec rails db:migrate

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[katsuyan]

@googlebot I signed it!

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