Issue #3231992: Make the staged sites/default writable before cleaning

src/ComposerStager/Cleaner.php

@@ -57,7 +57,7 @@ class Cleaner implements CleanerInterface {
   /**
    * {@inheritdoc}
    */
-  public function clean(string $stagingDir, ?ProcessOutputCallbackInterface $callback, ?int $timeout = 120): void {
+  public function clean(string $stagingDir, ?ProcessOutputCallbackInterface $callback = NULL, ?int $timeout = 120): void {
     // Ensure that the staged site directory is writable.
     $site_dir = implode(DIRECTORY_SEPARATOR, [
       $stagingDir,