[FIX] purchase: correct currency for purchase_order_line

If the currency_id and cost_currency_id is different for a product, the cost in the purchase order is wrong. The purchase order should use cost_currency_id instead of currency_id.

Steps to reproduce on runbot:
-Set the currency of main company as Dollars (USD).
-Choose in runbot the company "My Belgian Company" as its currency is Euro.
- Add the currency rate of today between EUR and USD
- Take a product and edit the cost to 100 EUR - as the cost field is linked with the currency of the company
make sure on the tab purchase there is no information - so the vendor pricelist is not present.
- Go inside the purchase module and start a request for a quotation
leave the currency as EUR
- Select the product to purchase

Current Behavior:
The unit price will be (100*currency rate). If you do the same but take the currency as USD the product unit price on the purchase order line will be 100

Expected Behavior:
The unit price should have been 100 Euros. And when dollar is chosen as currency, the exchange should have been applied.

OPW-3460551

closes odoo/odoo#133024

Signed-off-by: Tiffany Chang (tic) <tic@odoo.com>