release: Always dist sets on dvd1

In the !NOPKGBASE && !NODISTSETS case, we only want the pkgbase repo
on disc1; but we want both packaging formats on dvd in order to allow
for offline installs in both methods and also tools which ingest the
dist sets from the dvd image.

MFC after: 12 hours
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52537