Issue #3230507: Create build tests for case where site is using core recommended Composer project

src/Updater.php

@@ -124,7 +124,8 @@ class Updater {
     // @todo This should be unique, in order to support parallel runs, or
     // multiple sites on the same server. Find a way to make it unique, and
     // persistent for the entire lifetime of the update process.
-    return FileSystem::getOsTemporaryDirectory() . '/.automatic_updates_stage';
+    $site_id = \Drupal::config('system.site')->get('uuid');
+    return FileSystem::getOsTemporaryDirectory() . '/.automatic_updates_stage_' . $site_id;
   }
 
   /**