Add BUILD_BROKEN_PYTHON_IS_PYTHON2

This provides an escape hatch to undo the effects of
aosp/2417672. It's an enviornment variable instead of
a product config variable because python is often used
during product config.

Bug: 203436762
Test: m nothing and get_build_var with aosp/2458935
Change-Id: I0dd0f7c221427ea487fd7993aef68ce803eaff21