[FIX] purchase_stock: valuation layer wrong when different currency

To reproduce:
1. Create a storable product with ordered quantities policy for purchase and with a category set as FIFO automated
2. Create a purchase order in a different currency and set quantities for partial deliveries
3. Create a partial delivery
4. Fully invoice the purchase order
5. Create the delivery of the complementary units
6. Check the stock valuation layer
The valuation of backorder is wrong.

The value on valuation layer is in company's currency, while the value
on invoice line is in the PO's currency. When create valuation line for
backorder, we didn't convert them to same currency.

opw-3300266

closes odoo/odoo#123118

Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>