refactor: remove dead updater and simplify ISO launcher

iso/builder/test-squashfs-layers.sh

@@ -16,6 +16,11 @@ for tool in mksquashfs unsquashfs rsync; do
     }
 done
 
+find . -maxdepth 0 -printf '' >/dev/null 2>&1 || {
+    echo "test-squashfs-layers: GNU find not available, skipping"
+    exit 0
+}
+
 T="$(mktemp -d)"
 trap 'rm -rf "$T"' EXIT INT TERM HUP
 ROOT="$T/root"