This reinstates a modified version of
b08439809f0868a74d0bd0e14d45cb3e5dd46a8c, which contained a thinko.
Previously, build-time metadata from (guix config) would be ignored when
available--e.g., when running /run/current-system/profile/bin/guix.
This is a followup to 316fc2acbb112bfa572ae30f95a93bcd56621234.
* guix/scripts/system/reconfigure.scm (check-forward-update): Use
'current-channels' rather than 'current-profile' + 'profile-channels'.
270e1b9e1e