Fix button hover border (#30048)

Fix regression from https://github.com/go-gitea/gitea/pull/30014. The
rule was to broad and affecting things like `primary` button
unintentionally.

(cherry picked from commit bbaf62589fe538be4afc86455d772360de80e7d8)

web_src/css/modules/button.css

···
 .ui.button:focus {
   background: var(--color-hover);
   color: var(--color-text);
-  border-color: var(--color-secondary-dark-2);
 }
 
 .page-content .ui.button {
···
 
 .ui.buttons .button {
   border-right: none;
+}
+
+.ui.buttons .button:hover {
+  border-color: var(--color-secondary-dark-2);
 }
 
 .ui.buttons .button:hover + .button {