mackup: use PYTHONPATH wrapper

Problems with the current mackup formula were reported in lra/mackup#330
and pull request #32833 was submitted by lra to address them.  However,
it was suggested to use a different approach that is more standard for
stand-alone Python scripts.  This patch applies the suggested changes.

Closes #32967.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>