[FIX] point_of_sale: Record in for is used

The method ``action_pos_order_invoice`` is multi, and has a ``for`` to
each record, then, inside of that for, is called the method
``_action_create_invoice_line``, but is called with self, not with
order, that is each record.

This was fixed to avoid singleton error.

closes odoo/odoo#38809

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>