child_process: set shell to false in fork()

This commit ensures that spawn()'s shell option is unconditionally
set to false when fork() is called.

Refs: nodejs/node#15299
Fixes: nodejs/node#13983
PR-URL: nodejs/node#15352
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>