go-dep: Avoid use of 'go mod' support

dep utility must not use 'go mod' support, so we explicitly disable it.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b34000ae)
Signed-off-by: Steve Sakoman <steve@sakoman.com>