[FIX] purchase_stock: price difference currency conversion date

Have a product P configured with:
* Product Type: Storable Product
* Product category:
    * costing method: standard
    * Inventory Valuation Automated
    * Price Difference Account: "500000 Cost of Goods Sold"
* Cost $100

Activate Multicurrency:
* USD main currency, CZK foreign currency
* On CZK set the rates
    * date1, Unit per USD: 30
    * date2, Unit per USD: 25 (date2 > date1)
    * date3, Unit per USD: 26 (date3 > date2)

Create a purchase order:
* Date: date2
* Currency: CZK
* Order line:
    * Product P
    * Quantity 10
    * Unit price 3000
Confirm, receive product on date2 (svl needs to be in date2)
Create the vendor bill with:
* Bill date: date 1
* Accounting date: date 3
Confirm the bill

Issue

Check the bill journal items. Price difference account is debited with 4000 czk.
This amount is not correct.
The valuation layer price unit is 100$
At the receipt date the price unit should be 2500czk
The price unit difference 3000-2500 = 500czk
Price difference 500*10 = 5000czk
The issue occur because the price difference unit is converted at bill
accounting date, resulting in 4000czk

opw-3063809

closes odoo/odoo#108180

Signed-off-by: William Henrotin (whe) <whe@odoo.com>