[FIXED JENKINS-18918] From plugins we must use target/ not target/classes/ to unpack Maven.

Also cleaning up variable names to be less misleading.

Note that new File("target") is really a bad idea since we cannot predict the CWD in effect when Maven is run.
(From the standard bin/mvn launcher, it seems that it will always be *some* Maven project at least.
This may not be true for Maven run embedded from various tools.)
But leaving that as is for now, since it is not clear how else we would find the project location.
(new File(env.description().getTestClass().getProtectionDomain().getCodeSource().getLocation().toURI()).getParentFile() might work but it is pretty hacky.)