Log Message:
python{39,310}: fix the build when the work directory is in $PREFIX

As documented in pkg/56774, when WRKOBJDIR is in LOCALBASE (eg set to
${LOCALBASE}/work) then changes done to Python's setup.py made it
unable to locate its own built-in modules, then failing to bootstrap and
build.

As suggested by tnn@; tested on NetBSD/amd64.

XXX pull-up to pkgsrc-2022Q2