release: ensure default sudo flavor is selected

pkg behaviour selects the non-default flavour, this trims
0,5GiB per cloudware image, by being explicit.

Approved by: cperciva
Differential Revision: https://reviews.freebsd.org/D48599
Sponsored by: SkunkWerks, GmbH
PR: 284278
MFC after: 10 days

(cherry picked from commit 457d745d90a6b63793c9ca17bcd7ec5ff4cd8b93)