[3.13] gh-143706: Fix sys.argv not set during multiprocessing forkserver `__main__` preload (GH-143717) (#143821)

The forkserver was not passing sys.argv to its main() function, causing
sys.argv to be empty during `__main__` module import in child processes. This
fixes a non-obvious regression inadvertently introduced by the gh-126631 main
preloading fix.

(cherry picked from commit 298d5440eb83f2dfd5651bac86d1592ec358d54c)