[FIX] purchase: Supplier currency in Vendor bill

Steps to reproduce the bug:

- Let's consider a vendor V with a supplier currency €
- Let's consider the current company's user C in $
- Let's consider two vendor bills journals J1 in € and J2 in $ from C
- Create a vendor bill for V

Bug:

The default currency was $ instead of € and the default journal was J2 instead
of J1. This fix keeps the same behavior as implemented in 82794c86

opw:2226115

closes odoo/odoo#48669

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>