ci: build musl only for arm64 and reuse toolchain cache [CI]

ci/cleanup-build-cache.sh

@@ -2,6 +2,10 @@
 # Caller holds its store lock. Keep recent image layers and persistent RUN caches.
 set -euo pipefail
 : "${CONTAINERS_STORAGE_CONF:?an explicit, locked CI Buildah store is required}"
+# No build is active while the caller holds the exclusive store lock. Remove
+# working containers left by canceled builds before considering old image layers.
+buildah unshare buildah rm --all >/dev/null 2>&1 || true
+buildah rmi localhost/moshiro-agent-binaries:linux-musl-x64 >/dev/null 2>&1 || true
 # prune also deletes RUN cache mounts; remove only expired intermediate layers and versioned application images.
 mapfile -t expired < <(buildah images --all --no-trunc --format '{{.ID}} {{.Created}} {{.Name}}:{{.Tag}}' | python3 -c '
 import re,sys,time