Added permisison check. Maybe fix for JENKINS-19757

There are jenkins ticket for copyArtifact plugin JENKINS-11655, where blamed that plugin copy wrong artifacts. I didn't finad any similar issue for Build Promotion plugin, even it have the same problem. When there are used "Downsteam trigger" in promotion criteria, and downstream build is started by more than one build, first build is promoted when job is done. In this change latest build is promoted. 

BUT, if you are using Copy Artifact plugin in downstream job, job where Artifacts are copied will be promoted.
There is need to fix this issue in Copy Artifact plugin to get it work also like this promotion plugin.