Use Variable for border-radius

The border-radius was hard coded, so i changed it to: @button__border-radius

Author: Scarbous

app/design/frontend/Magento/blank/web/css/source/_extends.less

@@ -1110,7 +1110,7 @@
     .abs-shopping-cart-items {
         .action {
             &.continue {
-                border-radius: 3px;
+                border-radius: @button__border-radius;
                 font-weight: @font-weight__bold;
                 .lib-link-as-button();
                 .lib-button(