[FIX] purchase: use the vendor currency as default currency for vendor bills

Create a new vendor bill.
Choose a vendor with a "supplier currency" different from the company default.
The currency on the vendor bill does not change to the currency of the vendor,
unlike when a purchase order is created.

We set the currency in the partner_id onchange.

opw 1932683

closes odoo/odoo#30961

addons/purchase/models/account_invoice.py

@@ -146,6 +146,8 @@ class AccountInvoice(models.Model):
                 self.journal_id = default_journal_id
             if self.env.context.get('default_currency_id'):
                 self.currency_id = self.env.context['default_currency_id']
+            if self.partner_id.property_purchase_currency_id:
+                self.currency_id = self.partner_id.property_purchase_currency_id
         return res
 
     @api.model