Change border-style: none to border-style: 0 as preferred by linting

wagtail/admin/static_src/wagtailadmin/scss/layouts/workflow-progress.scss

@@ -10,7 +10,7 @@
 
     &__button {
         background: none;
-        border-style: none;
+        border-style: 0;
         padding-bottom: 5px;
         font-weight: bold;