test/system: Silence SC2004

Otherwise https://www.shellcheck.net/ would complain
  Line 110:
  for ((i = ${num_of_retries}; i > 0; i--)); do
            ^---------------^ SC2004 (style): $/${} is unnecessary on
                              arithmetic variables.

See: https://www.shellcheck.net/wiki/SC2004

#1347

test/system/libs/helpers.bash

@@ -107,7 +107,7 @@ function _pull_and_cache_distro_image() {
     assert_success
   fi
 
-  for ((i = ${num_of_retries}; i > 0; i--)); do
+  for ((i = num_of_retries; i > 0; i--)); do
     run $SKOPEO copy --dest-compress docker://${image} dir:${IMAGE_CACHE_DIR}/${image_archive}
 
     if [ "$status" -eq 0 ]; then