test/system: Silence SC2154

Otherwise https://www.shellcheck.net/ would complain:
  Line 343:
  if [ "$status" -ne 0 ]; then
        ^-----^ SC2154 (warning): status is referenced but not assigned.

See: https://www.shellcheck.net/wiki/SC2154

https://github.com/containers/toolbox/pull/1378

test/system/libs/helpers.bash

@@ -340,6 +340,8 @@ function pull_distro_image() {
 
   # https://github.com/containers/skopeo/issues/547 for the options for containers-storage
   run "$SKOPEO" copy "dir:${IMAGE_CACHE_DIR}/${image_archive}" "containers-storage:[overlay@$ROOTLESS_PODMAN_STORE_DIR+$ROOTLESS_PODMAN_STORE_DIR]${image}"
+
+  # shellcheck disable=SC2154
   if [ "$status" -ne 0 ]; then
     echo "Failed to load image ${image} from cache ${IMAGE_CACHE_DIR}/${image_archive}"
     assert_success