Fix btn-primary in flash alerts (#1991)

* Fix btn-primary in a flash alert

* Create cuddly-mugs-peel.md

Author: simurai

.changeset/cuddly-mugs-peel.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Fix `btn-primary` in flash alerts

src/alerts/flash.scss

@@ -63,8 +63,12 @@
     color: var(--color-fg-muted);
   }
 
-  &.btn-primary .octicon {
-    color: inherit;
+  &.btn-primary {
+    background-clip: border-box;
+
+    .octicon {
+      color: inherit;
+    }
   }
 }