Remove explicit setting of PYTHON in CI jobs #2576
Description
As a follow on to #2507, even with Python 3 available on the CI hosts a lot of the test jobs are explicitly setting PYTHON=python before running the makefile. This issue will track undoing the explicit set on the basis that it is redundant on existing release lines as the default value of PYTHON in the makefile if not set in the environment or via running configure is python.
Refs: nodejs/node#36691
Refs: nodejs/node#37764
Refs: #2507
