tweak gpg sign plugin for running on new Jenkins CI
tweak gpg sign plugin for running on new Jenkins CI

As documented in https://wiki.eclipse.org/Jenkins#How_can_artifacts_be_deployed_to_OSSRH_.2F_Maven_Central.3F

eclipse/rdf4j,eclipse/rdf4j,eclipse/rdf4j,eclipse/rdf4j,eclipse/rdf4j,eclipse/rdf4j