archivers/py-borgbackup: use spawn for multiprocessing

borgbackup's setup.py fails with Python 3.14's new forkserver default
(borgbackup predates the Python 3.14 release)

Note "make test" is currently unavailable on Python 3.13 and 3.14
becuase it expects py-sphinx, but the sphinx 5 version currently in
ports uses imghr, which was removed in 3.13.