Fix pytest detection in case `which' fails.
Fix pytest detection in case `which' fails.

probcomp/crosscat,probcomp/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat