[JENKINS-16089] Revising 88feabb4

Based on comments from Jesse, revising the fix.

I'm now putting permlinks inside the builds/ directory to avoid the computing hassle involved in the split $JENKINS_HOME.

What we historically had in $JENKINS_HOME/jobs/JOB/lastSuccessfulBuild is also now subsumed by this feature. I initially attempted to create these permalinks in the buidl root directory, but turns out those symlinks aren't the same name as the ID of permalinks, so it doesn't mesh well.

And finally, a test!