[ticket/16688] Fix issues with custom repositories

Make any custom repo non-canonical to give repo.packagist.org (if enabled)
higher repository priority in case custom repo contains outdated/incorrect
packages/dependencies.

PHPBB3-16688

phpBB/phpbb/composer/installer.php

@@ -591,6 +591,7 @@ class installer
 				$repositories[] = [
 					'type' => 'composer',
 					'url' => $repository,
+					'canonical' => $this->packagist ? false : true,
 				];
 			}
 		}