JAVA_HOME is set as necessary in packaging tests, depending on whether
it is needed for no-jdk distributions or testing override behavior. We
currently rely on gradle finding java through PATH. However, JAVA_HOME
can sometimes be set by the system itself, which then leaks through to
the packaging test. This commit reworks our handling of JAVA_HOME to
pass it through for gradle, and then explicitly clear it whenever
running shell commands in packaging tests.