Add GitHub credentials for https://github.com/eclipse-platform/eclipse.platform.releng.aggregator to https://ci.eclipse.org/platform/

This is the next step now #787 (closed) is resolved. Some Platform builds do run on https://ci.eclipse.org/platform/ . Can you please install there the same GitHub account for eclipse-platform repositories on GitHub than the one that's installed on ci.eclipse.org/releng?