build: skip the database check when generating install media

There are several scripts and targets solely used to generate install
media, make sure DB_FROM_SRC is used in that case in order to prevent
checking the host database, which is irrelevant when generating
install binaries.

Sponsored by: Citrix Systems R&D
PR: 230459
Reviewed by: gjb
Differential revision: https://reviews.freebsd.org/D16638