Use --color-border-secondary for Box-row dividers

Author: simurai

src/box/box.scss

@@ -106,7 +106,7 @@
   // stylelint-disable-next-line primer/spacing
   margin-top: -1px;
   list-style-type: none; // To account for applying Box component to a list
-  border-top-color: var(--color-border-primary);
+  border-top-color: var(--color-border-secondary);
   border-top-style: $border-style;
   border-top-width: $border-width;