[FIX] pos_restaurant: distinguish product variants

When using a product attribute with a creation mode defined on "never",
the order sent to the kitchen doesn't consider the product attribute
value

To reproduce the issue:
(Use demo data)
1. Create a product P:
    - In variants, add the "size" attribute with at least two values
        - (Note that the creation mode of this attribute is "never")
    - Available in POS
        - Category: Miscellaneous
2. In Point of Sale, edit the "Bar" POS and enable:
    - Product Configurator
    - Order Printer (the Kitchen Printer must be working)
3. Start a POS session
4. Select a table and add:
    - 1 x P with a size value
    - 1 x P with another(!) size value
5. Send the order to the kitchen

Error: The printed order only contains one line: 2 x P with the first
size value selected

When checking if the current order has some changes, the lines summary
builrder does not distinguish the product variants

OPW-2698626

closes odoo/odoo#81660

Signed-off-by: Masereel Pierre <pim@odoo.com>