The builder stage never declared ARG MIRROR, so the keyring [omarchy] repo
pointed at the channel-less legacy pkgs.omarchy.org/$arch path — it works
only because a stale copy of the old layout still answers there, and it would
miss a keyring rotation. Each image now pulls omarchy-keyring from its own
channel (edge/rc/stable), matching the base mirror it already selects.
update-repo and remove-package switch to the edge x86_64 image: repo-add and
repo-remove compile nothing, and using the channel image would deadlock
bootstrap-rc — the rc image can only build once the rc channel it pulls the
keyring from exists remotely.