Use injected file_system service instead of \Drupal::service()

ConfigImporterService::copy() call introduced in !16 (merged) used \Drupal::service('file_system'), which triggers PHPStan globalDrupalDependencyInjection.useDependencyInjection.

The class already receives @file_system through its constructor and stores it in $this->fileSystem, so this switches the call to $this->fileSystem->copy(...). No constructor or services.yml change; no BC impact.