check-invariants: Do not complain about closed channels
When we release a channel like apertis/v2023dev0
we still want to keep
it monitored for errors to catch issues that may impact downstreams when
they mirror it, but we do not want to hear complaints about packages
missing updates and lagging behind as that's on purpose.
This clears the dashboard from 685-564 = 111 actually-not-errors.
Edited by Emanuele Aina