Don't rebuild core when already on maven.

PR validation can be retried with PLS REBUILD.
As our artifactory does not allow overwriting snapshots,
rebuilds would fail if artifacts are already published.

With this tweak, PLS REBUILD works and is faster to boot!

Tested by https://scala-webapps.epfl.ch/jenkins/view/pr-validators/job/pr-scala-publish-core/28/console