release: Add missing dependencies in the minimal OCI image

The pkg-bootstrap binary depends on fetch.

Approved by: re
Reviewed by: cperciva, emaste, bapt
Differential Revision: https://reviews.freebsd.org/D47661
MFC after: 1 day

(cherry picked from commit ca1f1d2c9419a5d66929ca6880b69901254645a6)