[SPARK-10932] [PROJECT INFRA] Port two minor changes to release-build.sh from scripts' old repo

Spark's release packaging scripts used to live in a separate repository. Although these scripts are now part of the Spark repo, there are some minor patches made against the old repos that are missing in Spark's copy of the script. This PR ports those changes.

/cc shivaram, who originally submitted these changes against https://github.com/rxin/spark-utils

Author: Josh Rosen <joshrosen@databricks.com>

Closes #8986 from JoshRosen/port-release-build-fixes-from-rxin-repo.