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: https://github.com/nodejs/node/pull/15299
Fixes: https://github.com/nodejs/node/issues/13983
PR-URL: https://github.com/nodejs/node/pull/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>