[BUILD] Always run SQL tests in master build.

Seems our master build does not run HiveCompatibilitySuite (because _RUN_SQL_TESTS is not set). This PR introduces a property `AMP_JENKINS_PRB` to differentiate a PR build and a regular build. If a build is a regular one, we always set _RUN_SQL_TESTS to true.

cc JoshRosen nchammas

Author: Yin Huai <yhuai@databricks.com>

Closes #5955 from yhuai/runSQLTests and squashes the following commits:

3d399bc [Yin Huai] Always run SQL tests in master build.