Button: fix style of the only button in a button group (#9298)

packages/theme-chalk/src/button.scss

@@ -188,6 +188,12 @@
       border-top-left-radius: 0;
       border-bottom-left-radius: 0;
     }
+    &:first-child:last-child {
+      border-top-right-radius: $--button-border-radius;
+      border-bottom-right-radius: $--button-border-radius;
+      border-top-left-radius: $--button-border-radius;
+      border-bottom-left-radius: $--button-border-radius;
+    }
     &:not(:first-child):not(:last-child) {
       border-radius: 0;
     }