Force multiprocessing method to be fork if available

Since Python 3.8, the multiprocessing module defaults to the spawn
method on MacOS, since the fork method is considered unsafe.

However, BuildStream is not compatible with spawn mode so we
force the default to fork (if available) which works for the most part.

See
https://lists.apache.org/thread.html/r08edd3dd10ce3039da5eb01f0722d25c44406ab8afadcc3ae78a5414%40%3Cdev.buildstream.apache.org%3E
for some related discussion.
1 file changed