fixed a bug in computing classworlds jar location.

See http://www.nabble.com/linux-slave-uses-wrong-maven-path-for-classworlds-jar-tf4648221.html


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@5312 71c3de6d-444a-0410-be80-ed276b4c234a