[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.
macos_multiprocess_workaround in 4 minutes and 25 seconds (queued for 1 second)4 jobs for