Improve cache usage

1. Only download cached files that are not present in the current cache
2. Try pulling directly from the cache, not only with the target
   information

.gitlab-ci.yml

@@ -77,7 +77,9 @@ downloads:
     - |-
       for vars in $(find distros/ -name '*.pkrvars.hcl'); do
         file=$(echo var.iso.url | packer console -var-file=${vars} -config-type=hcl2 sources/)
-        files+=("-O" "${file}")
+        if [ ! -e "cache/${file}" ]; then
+          files+=("-O" "${file}")
+        fi
       done
     - mkdir -p cache
     - pushd cache