[hotfix] [build] Make sure JAVA_HOME is set for tests.

This is needed at least for YARN integration tests, since `$JAVA_HOME` is used to launch the executors.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5441 from vanzin/yarn-test-test and squashes the following commits:

3eeec30 [Marcelo Vanzin] Use JAVA_HOME when available, java.home otherwise.
d71f1bb [Marcelo Vanzin] And sbt too.
6bda399 [Marcelo Vanzin] WIP: Testing to see whether this fixes the yarn test issue.