Button block: reduce chance of style conflicts. (#24919)

Follow up to #24599.

Using the child combinator (instead of the descendant combinator) narrows the selector without increasing its specificity. This should help reduce the likelihood of a style issue occurring when a parent element has an `is-style-outline` CSS class.

https://developer.mozilla.org/en-US/docs/Web/CSS/Child_combinator

packages/block-library/src/button/style.scss

@@ -47,7 +47,7 @@ $blocks-button__height: 3.1em;
 	border-radius: 0 !important;
 }
 
-.is-style-outline .wp-block-button__link,
+.is-style-outline > .wp-block-button__link,
 .wp-block-button__link.is-style-outline {
 	color: #32373c;
 	background-color: transparent;