RCORE-2134 GHA release script fixes

[ironage]

Related to #7566 and #7726.
Fixes based on the error reported in this run.

The error is

Rebasing commits made to branch 'master' on to base branch 'refs/heads/master'
  fatal: refusing to fetch into branch 'refs/heads/master' checked out at '/home/runner/work/realm-core/realm-core'

This is because I had set the base parameter unnecessarily, and the action wasn't smart enough to know that it should be a no-op. According to the docs:

There may be use cases where it makes sense to execute the workflow on a branch that is not the base of the pull request. In these cases, the base branch can be specified with the base action input. The action will attempt to rebase changes made during the workflow on to the actual base.