Revert "macos: use posix_spawn instead of fork"

This reverts commit 39968db.

This commit caused a number of failures in the Node.js test suite
while attempting to release libuv 1.41.0.

Refs: libuv#3086
Refs: libuv#3064
PR-URL: libuv#3107
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>