[SPARK-3355][Core]: Allow running maven tests in run-tests

Added an AMPLAB_JENKINS_BUILD_TOOL env. variable to allow differentiation between maven and sbt build / test suites. The only issue I found with this is that, when running maven builds I wasn't able to get individual package tests running without running a `mvn install` first. Not sure what Jenkins is doing wrt its env., but figured its much better to just test everything than install packages in the "~/.m2/" directory and only test individual items, esp. if this is predominantly for the Jenkins build. Thoughts / comments would be great!

Author: Brennon York <brennon.york@capitalone.com>

Closes #4734 from brennonyork/SPARK-3355 and squashes the following commits:

c813d32 [Brennon York] changed mvn call from 'clean compile
616ce30 [Brennon York] fixed merge conflicts
3540de9 [Brennon York] added an AMPLAB_JENKINS_BUILD_TOOL env. variable to allow differentiation between maven and sbt build / test suites