gh-90622: Do not spawn ProcessPool workers on demand via fork method. (GH-91598) (#92495)

Do not spawn ProcessPool workers on demand when they spawn via fork.

This avoids potential deadlocks in the child processes due to forking from
a multithreaded process.
(cherry picked from commit ebb37fc3fdcb03db4e206db017eeef7aaffbae84)

Co-authored-by: Gregory P. Smith <greg@krypto.org>