[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
Normally this should not have to be called from plugins, because `AbstractBuild.getEnvironment(TaskListener)` calls it already.