Coding Standards: Remove an extra variable in get_author_posts_url().

This fixes a `Variable "$auth_ID" is not in valid snake_case format` WPCS warning by using the existing `$author_id` variable, and brings consistency with a similar fragment in `get_author_feed_link()`.

Follow-up to [979], [5087], [6364], [6365].

See #54728.

git-svn-id: https://develop.svn.wordpress.org/trunk@52568 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/author-template.php

@@ -343,12 +343,13 @@ function the_author_posts_link( $deprecated = '' ) {
  */
 function get_author_posts_url( $author_id, $author_nicename = '' ) {
 	global $wp_rewrite;
-	$auth_ID = (int) $author_id;
-	$link    = $wp_rewrite->get_author_permastruct();
+
+	$author_id = (int) $author_id;
+	$link      = $wp_rewrite->get_author_permastruct();
 
 	if ( empty( $link ) ) {
 		$file = home_url( '/' );
-		$link = $file . '?author=' . $auth_ID;
+		$link = $file . '?author=' . $author_id;
 	} else {
 		if ( '' === $author_nicename ) {
 			$user = get_userdata( $author_id );