[FIX] purchase_stock: stock valuation foreign currency Journal Entry

Steps to reproduce:
- Setup product (
    cost 10$
    Storeable Product
    Standard Price Accounting
    automated valuation
    add account for price difference
)

- Purchase product in foreign currency (set a different price)
- Receive the product
- Create and validate Bill

Bug:
The reception JE is for the cost amount set on the product ($10)
debit and credit value are correct but the amount in currency
is wrong (purchase cost)

After creating the bill the Journal items are not correctly matched
since reconciliation is done on currency amount in the case fo foreign
currency transaction

Fix:
Set the correct amount (cost configured on product page)
in currency amount if costing method is standard

opw-2822366

closes odoo/odoo#101809

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