Always include locale in the mediables pivot

[bonroyage]

Description

#2320 added support for copying the locale when duplicating models with mediables, but since the locale isn't included in the pivot when the config was false, this would have caused the value from the pivot to be null and thus causes the tests to fail.

The fix is to always include locale in the pivot, regardless of the config.

Illuminate\Database\QueryException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'locale' cannot be null (Connection: mysql, SQL: insert into `twill_mediables` (`created_at`, `crop`, `crop_h`, `crop_w`, `crop_x`, `crop_y`, `locale`, `media_id`, `mediable_id`, `mediable_type`, `metadatas`, `ratio`, `role`, `updated_at`) values (2023-11-03 08:57:44, default, ?, ?, ?, ?, ?, 1, 2, App\Models\XBleafe, [], default, cover, 2023-11-03 08:57:44))