Restore `$soucrce` in `WP_Upgrader::install_package()`.

See #22400.


git-svn-id: https://develop.svn.wordpress.org/trunk@28467 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-upgrader.php

@@ -190,6 +190,7 @@ class WP_Upgrader {
 		);
 
 		$args = wp_parse_args($args, $defaults);
+		$source = $args['source'];
 		$destination = $args['destination'];
 		$clear_destination = $args['clear_destination'];