[FIX] purchase: fiscal position from onchange_partner_id

Used case:

-Configure admin as multi-company user
-Create 2 fiscal positions (one for company "Odoo BE" and one for company "Odoo US")
-Set admin on company "Odoo BE"
-On supplier (Asustek) configure fiscal position Odoo BE
-Set admin on company "Odoo US"
-On supplier (Asustek) configure fiscal position Odoo US
-Configure a product (Laptop E5023) with:
   *route "Buy"
   *supplier (Asustek) without company
   *reordering rules (min qty: 20, max qty: 40)
-Set admin on company "Odoo BE"
-Run scheduler via the cron

Behavior before the fix:

-Fiscal position on the created PO is the fiscal position for "Odoo BE" (and PO is for the company "Odoo US")

Behavior after the fix:

-Fiscal position on the create PO is the fiscal position for "Odoo US".

Closes #11537

opw:673288