release: Add pkg .conf to METALOG on dvd

We're correctly recording all of the packages in the dvd METALOG file,
but if we don't record ./packages/repos/FreeBSD_install_cdrom.conf then
users won't be able to install them very easily.

Reviewed by: markj
Reported by: Lars Tunkrans
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D53199