Adjust Jenkinsfile for move of releng.aggregator to github

Change-Id: I7c5a0b70fda66207784e20c0f118f4b4ecf10e29
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.releng/+/190629
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>