gh-129204: Add _PYTHON_SUBPROCESS_USE_POSIX_SPAWN environment knob (GH-132184)

* Add _PYTHON_SUBPROCESS_USE_POSIX_SPAWN environment knob

Add support for disabling the use of `posix_spawn` via a variable in
the process environment.

While it was previously possible to toggle this by modifying the value
of `subprocess._USE_POSIX_SPAWN`, this required either patching CPython
or modifying it within the interpreter instance which is not always
possible, such as when running applications or scripts not under a
user's control.

Signed-off-by: Vincent Fazio <vfazio@gmail.com>

* fixup NEWS entry

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Signed-off-by: Vincent Fazio <vfazio@gmail.com>