`subprocess._USE_POSIX_SPAWN` necessary to make nested launching work for AppImage

[C0rn3j]

The release notes for 3.13 ask this - Please report your reason and platform details in the issue tracker if you set this so that we can improve our API selection logic for everyone. - so here I am.

I maintain sc-controller and Python 3.13 breaks its AppImage releases, as reported here AppImage/AppImageKit#1435 with some attempted troubleshooting.

I've had to set subprocess._USE_POSIX_SPAWN = False in C0rn3j/sc-controller@cb03aa8 to get around it.