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.

bin/build

@@ -164,11 +164,15 @@ mkdir -p "$BUILD_OUTPUT_DIR" "$REPO_DIR" "$SRC_DIR"
 # Check Docker is available
 check_docker
 
-# Setup QEMU for aarch64 builds on x86_64 hosts
-if [[ "$(uname -m)" == "x86_64" && "$ARCH" == "aarch64" ]]; then
-  # Check if QEMU is already working
-  if ! docker run --rm --platform linux/arm64 alpine:3.21 /bin/true >/dev/null 2>&1; then
-    print_info "Setting up QEMU for ARM64 emulation..."
+# A foreign target architecture runs under QEMU user emulation. Probe by
+# actually running a container for the target platform: that is the only
+# test that covers both "binfmt not registered" and "registered but broken".
+HOST_ARCH=$(uname -m)
+[[ "$HOST_ARCH" == "arm64" ]] && HOST_ARCH=aarch64
+if [[ "$HOST_ARCH" != "$ARCH" ]]; then
+  # shellcheck disable=SC2086
+  if ! docker run --rm $(get_platform_arg "$ARCH") alpine:3.21 /bin/true >/dev/null 2>&1; then
+    print_info "Setting up QEMU for $ARCH emulation on this $HOST_ARCH host..."
     setup_qemu
   fi
 fi