Themes: Ensure the theme roots cache is cleared when registering a theme directory.

Props soulseekah, johnbillion

Fixes #43228


git-svn-id: https://develop.svn.wordpress.org/trunk@42788 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -413,6 +413,8 @@ function register_theme_directory( $directory ) {
 		$wp_theme_directories[] = $untrailed;
 	}
 
+	wp_clean_themes_cache();
+
 	return true;
 }