devel/py-libzfs: fix build with nonstandard SRC_BASE after r520716
Upstream git rev d801c5f59f20a1889a4e4c6366c9d12f2ee50b2f changed the name
of the env variable used to indicate the location of the FreeBSD source
tree.
CC @sonicaj
Approved by: wg (maintainer)
MFH: 2020Q1
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D23126