Fix :mod:`multiprocessing` forkserver so that :data:`sys.argv` is correctly
set before ``__main__`` is preloaded. Previously, :data:`sys.argv` was empty
during main module import in forkserver child processes. This fixes a
regression introduced in 3.13.8 and 3.14.1. Root caused by Aaron Wieczorek,
test provided by Thomas Watson, thanks!
