[FIX] stock: avoid attempt to create the same lot

How to reproduce:
  - Create a product tracked by lot;
  - Create two receipts for this product;
  - Use the same lot name in the two receipts;
  - Try to validate the two receipts in the same time
    => `ValidationError` will trigger in the `_check_unique_lot`
        constrain of the production lot.

task-2646107

Part-of: odoo/odoo#76847