python3Packages.pytest-xdist: Disable fork safety on darwin

We run pytest with `--forked` in nixpkgs, to reduce side effects that
can occur when multiple tests mutate their environment in incompatible
ways.

Forking on macOS 10.13 and later is unsafe when an application does work
between calls to fork() and its followup exec(). This may lead to
crashes when calls into the Objective-C runtime are issued, which will
in turn coredump the Python interpreter.

One good reproducer for this scenario is when the urllib module tries
to lookup proxy configurations in `urllib.request.getproxies()` through
`get_proxies_macos_sysconf` into the native `_scproxy` module.

This is a class of issues that is of course not limited to the urllib
module. The general recommendation is to use `spawn` instead of `fork`,
but we don't have any influence on upstream developers to do one or the
other.

One often cited workaround would be to disable fork safety entirely on
calls to `initialize()`, which is probably a better solution than
running without multithreading (slow) or without the `--forked` (prone
to side effects) mode.

This currently happens on aarch64-linux only, where we use more recent
11.0 SDK version, while x86_64-darwin has been stuck on 10.12 for a
while now.

https://github.com/python/cpython/issues/77906#issuecomment-1093788352
http://www.sealiesoftware.com/blog/archive/2017/6/5/Objective-C_and_fork_in_macOS_1013.html

Closes: #194290

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