Fix small gap in style variation button (#12090)

This PR fixes #12066.

I believe the issue is caused by the border on the button being an inset shadow, which stacks below the background colors of the content inside the preview. By making it a border, this becomes a non issue.

Author: jasmussen

packages/editor/src/components/block-styles/style.scss

@@ -29,7 +29,7 @@
 
 .editor-block-styles__item-preview {
 	outline: $border-width solid transparent; // Shown in Windows High Contrast mode.
-	box-shadow: inset 0 0 0 1px rgba($dark-gray-900, 0.2);
+	border: 1px solid rgba($dark-gray-900, 0.2);
 	overflow: hidden;
 	padding: 0;
 	text-align: initial;