[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)
Status Name Job ID Coverage
  Build
passed build_linux_37 #225957
docker

00:02:24

73.0%
passed build_linux_38 #225958
docker

00:02:25

74.0%
passed build_macos_intel_37 #225955
intel macos

00:04:21

73.0%
passed build_macos_intel_38 #225956
macos intel

00:04:25

74.0%