devel/py-libzfs: fix build with nonstandard FreeBSD src dir

The current version of py-libzfs requires FREEBSD_SRC to be set during the
configure phase as well as during the build phase.

Approved by: wg (maintainer)
Reviewed by: wg
MFH: 2019Q4
Sponsored by: Axcient
Differential Revision: https://svnweb.freebsd.org/ports/branches/2019Q4/