[FIX] purchase: Saving PO with a foreign currency

Steps to reproduce the bug:
- Create a company C with currency = USD
- Create a vendor V with a supplier currency = EUR
- Create a PO for V with one line, the default currency is EUR
- Save

Bug:

The currency of the PO was USD when saving because the field currency is
in readonly when the state is draft.

opw:2035640

closes odoo/odoo#35144

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

addons/purchase/views/purchase_views.xml

@@ -169,7 +169,7 @@
                                 placeholder="Name, TIN, Email, or Reference"
                             />
                             <field name="partner_ref"/>
-                            <field name="currency_id" groups="base.group_multi_currency"/>
+                            <field name="currency_id" groups="base.group_multi_currency" force_save="1"/>
                         </group>
                         <group>
                             <field name="date_order"/>