Make aarch64 a first-class architecture in the scheduled pipeline

One list, PUBLISHED_ARCHES in helpers/paths.sh (default x86_64,
overridable with OMARCHY_ARCHES), now drives everything the repository
host schedules. check-versions compares PKGBUILDs against each
architecture's channel databases and writes one queue per channel and
architecture; auto-release works through the queues one architecture at
a time, each with its own backoff, so a failing build on one never
blocks the other; advance-channel --arch all re-runs an advance for every
published architecture and omarchy-release uses it for start and ship,
building the pinned pair once per architecture in its rc trigger; the
train observes channels through the reference (first) architecture
instead of a hard-coded x86_64. Queue and backoff files written under
the old per-channel names are treated as x86_64 until consumed.

Two things made an aarch64 builder image impossible to create: the
keyring bootstrap fetched omarchy-keyring from the target architecture's
own channel tree, which does not exist before that architecture has
published anything, and the QEMU probe only knew the x86_64-host,
aarch64-target case. The keyring (arch=any) now always comes from the
x86_64 tree, and the probe compares host and target architectures and
runs a container for the target platform.

clean-repo grouped versions with a regex that only knew any, x86_64 and
i686, so aarch64 packages would never have been pruned.

build/Dockerfile

@@ -108,11 +108,16 @@ RUN ln -sf /usr/lib/os-release /etc/os-release && \
 # Setup Omarchy keyring manually before adding repo (avoids keyserver trust issues)
 # Note: Repository is removed at the end since build scripts add it dynamically.
 # The keyring comes from this image's own channel (the bare /$arch path is a
-# stale legacy layout); %s keeps pacman's $arch literal while MIRROR expands.
+# stale legacy layout). It is always taken from the x86_64 tree, whatever the
+# image's own architecture: omarchy-keyring is an arch=any package, and the
+# x86_64 tree is the one that exists before a new architecture has published
+# anything. Bootstrapping from the target's own tree would make the first
+# aarch64 build depend on an aarch64 repository that only that build can
+# create.
 ARG MIRROR=edge
 RUN pacman-key --recv-keys 40DFB630FF42BCFFB047046CF0134EE680CAC571 --keyserver keys.openpgp.org && \
     pacman-key --lsign-key 40DFB630FF42BCFFB047046CF0134EE680CAC571 && \
-    printf '\n[omarchy]\nSigLevel = Optional\nServer = https://pkgs.omarchy.org/%s/$arch\n' "${MIRROR}" >> /etc/pacman.conf && \
+    printf '\n[omarchy]\nSigLevel = Optional\nServer = https://pkgs.omarchy.org/%s/x86_64\n' "${MIRROR}" >> /etc/pacman.conf && \
     pacman -Sy --noconfirm && \
     pacman -S --noconfirm omarchy-keyring && \
     pacman-key --populate omarchy && \