nuageinit: chmod sudoers directory instead of chmod (again) sudoers file

• Set mode of sudoers to 0440.

Reviewed by: bapt@, jlduran@
Approved by: bapt@, jlduran@
Differential Revision: https://reviews.freebsd.org/D52438

(cherry picked from commit a5cc9b7b96dcba4d3ee98f2eb58c3f389590ddf0)