[JENKINS-20884] Variable expansion in maven goals

I had a Jenkins job, where I needed to calculate goals to be executed in a shell script. So I two pre-build steps:
- shell script
- inject environment variables (EnvInject)

Then I wanted to put that variable into maven goals, like this:

```
-o clean install -pl ${some_variable}
```

This did not work. This change fixes that.

Would be happy to know if this is ok, or should be fixed in some other way.

Regards
Marcin
Also prebuilders like Groovy script calls .getEnvironment() and uses EnvVars that maybe not the same that were called in MavenModuleSetBuild. If there is no critical overrides in variables before, then additional call to  .getEnvironment(TaskListener) maybe a solution.