[test][prefix][algorithms] Workaround multiprocess changes in macOS Python 3.8

The default implementation for the multiprocess
changed from fork to spawn on macOS with Python
3.8. This merge request forces the use of fork
for the tests on macOS. The runner will anyway
use Linux so taking more time to solve this
particular issue would not make much sense.
4 jobs for macos_multiprocess_workaround in 4 minutes and 25 seconds (queued for 1 second)