pyzfs: Adapt python lib directory evaluation from ax_python_devel.m4

71216b91d281e7e58f5e29ca4d4553945e080fe9 introduced a regression
on debian/ubuntu systems during build.

The reason being, that building the RPM for pyzfs was using
a different library path than building the library itself.
This is now harmonized.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Martin Rüegg <martin.rueegg@metaworx.ch>
Closes #16155
Closes #17480