[FIXED JENKINS-23713] Allow BuildStep to work with non-AbstractProject

[Trello](https://trello.com/c/xAsq2mNn/11-serious-look-at-buildstep-interop) and [JENKINS-23713](https://issues.jenkins-ci.org/browse/JENKINS-23713)
Or would it be safer to introduce a new method (and update the usages in `getRun` and `index.jelly`)?