daisy-builder should specify destination GCS path

[hopkiw]

When we built the daisy-builder image, we didn't have a standard for finding the bucket with appropriate permissions with package outputs. So as interim, we parsed the daisy scratch dir from its stdout. We have now updated all the packagebuild workflows to take destination paths as input, so daisy-builder:main.sh should now:

• Check if GCS_OUT variable is set in metadata
• If so, set the appropriate variable when running the workflow
• If so, copy artifacts out of $GCS_OUT to artifacts/
  and, as today:
• If postsubmit, also copy artifacts to new package bucket to trigger package repo updates.