JTREG_HOME and JT_HOME can interfere with jtreg self tests

On my local setup, i have JT_HOME configured to point to a released version that's installed locally. I use it so that it can launch jtreg command when running tests in the JDK repo. On the same local setup, I also do some bug fixes for jtreg project itself and when running the jtreg self tests for the jtreg project using "bash make/build.sh test" I'm seeing that the launched tests end up picking up JT_HOME environment variable and thus the tests end up running against the released version instead of the locally changed jtreg build.