[ticket/17024] Add composer/installers to allowed plugins

PHPBB3-17024

phpBB/phpbb/composer/installer.php

@@ -496,6 +496,9 @@ class installer
 			'repositories' => $this->get_composer_repositories(),
 			'config' => [
 				'vendor-dir'	=> $this->packages_vendor_dir,
+				'allow-plugins'	=> [
+					'composer/installers' => true,
+				]
 			],
 			'minimum-stability' => $this->minimum_stability,
 		];