[FIX] point_of_sale: set fiscal position on scanning client barcode

On scanning customer barcode, POS updates pricelist, but not fiscal position.

Fix it by using method `updatePricelist`, which both updates pricelist and
fiscal position.

STEPS:

-On a customer, define a fiscal position, a price list and a barcode
-Go to the POS, in settings, define that several price lists & fiscal positions are possible
-open a session and scan your customer.

opw-3102568

closes odoo/odoo#108668

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>