Switch the dvd1.iso pkg(8) configuration to use the quarterly
branch, now that the package sets are now available. This
normally would have been after BETA1, however there were issues
preventing package builds to succeed.

Switch the default pkg(8) repository to the quarterly branch
as well, to prevent 'pkg upgrade' from downgrading packages
as the 12.0-RELEASE cycle progresses.

These changes will be reverted after releng/12.0 is branched.

This is a direct commit to stable/12.

MFC after: 2 weeks
MFC note: fake MFC timeframe as a reminder to revert
Approved by: re (delphij)
Sponsored by: The FreeBSD Foundation