Set GitHub push user as BUILD_USER_ID

[irissmann]

The GitHub plugin provides the id of the user who triggers a build by a push to GitHub. We would like to set this user in the environment variable BUILD_USER_ID.

[oleg-nenashev]

The current implementation may be very risky as a default behavior. Reflection code for just a single Cause. I would rather check if there are better options offered in SCM API Plugin

[marc-guenther]

I fully agree that this is an ugly hack. Unfortunately pushedBy is a private member with no getter :(
https://github.com/jenkinsci/github-plugin/blob/master/src/main/java/com/cloudbees/jenkins/GitHubPushCause.java
And this is useful information, which just simply isn't exposed anywhere.

EDIT: Ah well, I guess the intended way is to register ourselves here:

• https://github.com/jenkinsci/github-plugin/blob/150fc933aa397e7fa997434e61427db05de1007d/src/main/java/org/jenkinsci/plugins/github/webhook/subscriber/DefaultPushGHEventSubscriber.java#L107