Setting setupPyBuildFlags makes builder add an additional target
"build_ext" to setup.py arguments in addition to "bdisst_wheel".
pyarrow's setup.py recently started to try to redo build_ext again which
broke the build.
This replaces standard checkPhase using setup.py that uses pytest-runner
with an explicit call to pytest. One reason to do that is because
setup.py triggers another rebuild when asked to do "test". Another
reason is that there seems to be a conflict between possible imports for
pyarrow: `pwd`/pyarrow vs $out/pyarrow. By some unknown reason this
triggers an import error on hydra and ofborg, but not on my machine. The
solution here is to remove `pwd`/pyarrow, keep the tests and use direct
call to pytest (setup.py needs `pwd`/pyarrow). The added benefit is that
we are now testing what is installed in $out.
