Allow PHPStan Extension Installer

mixins.yml

@@ -20,6 +20,8 @@
     - composer config --no-plugins --no-interaction allow-plugins.drupal/core-vendor-hardening true
     # Disallowing plugin script.
     - composer config --no-plugins --no-interaction allow-plugins.php-http/discovery false
+    # Allow PHPStan Extension Installer
+    - composer config --no-plugins --no-interaction allow-plugins.phpstan/extension-installer true
 
 .customreferences:
   LanguageFiles: []