buildEnvironmentFor job shouldn't override slave's environment

Slave environment is set in https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/Job.java#L361, and the removed blocked was overriding it with master's environment, leading to incorrect environment provided to polling operations after implementation of [JENKINS-19042](https://issues.jenkins-ci.org/browse/JENKINS-19042)