This is a followup to JENKINS-17127

For a freestyle project with an "Invoke top-level Maven targets" build step and the Xvfb plugin enabled, the DISPLAY environment variable is not set when it already exists in the environment of the slave.

To reproduce:

• create a slave with an existing environment variable: DISPLAY=:99
  eg. by setting this in the .profile of the slave
• create a freestyle job with the attached config, which basically:
  • contains a pom which outputs the DISPLAY environment variable
  • creates two buildsteps, a shell which executes mvn and a Maven buildstep doing the same.

I get the correct output (eg. DISPLAY=:1) from the shell build step, and the wrong output (DISPLAY=:99) from the maven buildstep.

BTW, I also tried a Maven Job, and it works there, too. The only problem exists in a maven buildstep in a freestyle job.