[FIX] pos_coupon: allow coupon creation when generated from POS

When a coupon is generated from POS order, you get a traceback saying
you don't have access rights on "coupon.coupon" when you are only a pos
user.

So when we need to create the coupons and write on it, we are doing it
in sudo, to let the pos process generate the coupon.

Task-id: 2742528
Part-of: odoo/odoo#84544