fix(release): Fix post-release script so it operates on master (#887)

We've been leaving onpremise master with the latest release, instead of nightly builds for 2 releases now. Even if the post-release script runs, it bumped the versions to nightly on the release branch, making it effectively a no-op. This should be addressed in Craft via getsentry/craft#115 but until then, we need this extra line.