[Possible bug] Using correct java home in packaging tasks

[fourlastor]

While testing setting the java home via

runtime {
    javaHome.set("/path/to/preferred/java/home/different/from/project")
}

I had issues with running the JPackageTask, and I suspect it's due to how it gets the java home folder (which seems not to use BaseTask.getJavaHomeOrDefault()).

I think I have a fix here fourlastor-forks#1 but I haven't tested it (PR on my fork because I needed the fixes from the other PR and this was the simplest way to show the diff 😀)

I'm going to update this issue as soon as I get some time to test out the fix

[fourlastor]

Closing as I opened #143