[3.13] Fix os.posix_spawn() error handling (GH-142532) (#142581)

Fix os.posix_spawn() error handling (GH-142532)

Consistently use `goto exit;` in `py_posix_spawn()`.
(cherry picked from commit 8cfa351560487d472df4c7d841814801fbf57397)

Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>