Fix runcommand's proc_open to inherit parent environment
Fix runcommand's proc_open to inherit parent environment

This fixes #3729. proc_open will inherit the parent environment if you
pass NULL for the environment parameter
(http://php.net/manual/en/function.proc-open.php). I've verified that
this works properly by testing this fix against my wp-config that
contained environment variables.

wp-cli/wp-cli,wp-cli/wp-cli