release: Switch back to latest packages for now

Normally in BETAs the DVD images get packages from the latest
quarterly package build; but aarch64 does not yet have a quarterly
package set built for 15.x, and this made the BETA1 aarch64 builds
fail. Switch back to using packages from the "latest" package set.

Direct commit to releng/15.0; will be reverted before the release.

Approved by: re (cperciva)
Sponsored by: https://www.patreon.com/cperciva