[FIX] purchase, sale: display foreign VAT properly on pdf

When printing a sale order or a purchase order using a foreign VAT fiscal position, the domestic VAT was always used on the pdf report instead of the foreign one.

Part-of: odoo/odoo#79144

addons/purchase/report/purchase_quotation_templates.xml

@@ -3,6 +3,7 @@
 <template id="report_purchasequotation_document">
     <t t-call="web.external_layout">
         <t t-set="o" t-value="o.with_context(lang=o.partner_id.lang)"/>
+        <t t-set="forced_vat" t-value="o.fiscal_position_id.foreign_vat"/> <!-- So that it appears in the footer of the report instead of the company VAT if it's set -->
         <t t-set="address">
             <div t-field="o.partner_id"
             t-options='{"widget": "contact", "fields": ["address", "name", "phone"], "no_marker": True, "phone_icons": True}'/>

addons/sale/report/sale_report_templates.xml

@@ -3,6 +3,7 @@
 <template id="report_saleorder_document">
     <t t-call="web.external_layout">
         <t t-set="doc" t-value="doc.with_context(lang=doc.partner_id.lang)" />
+        <t t-set="forced_vat" t-value="doc.fiscal_position_id.foreign_vat"/> <!-- So that it appears in the footer of the report instead of the company VAT if it's set -->
         <t t-set="address">
             <div t-field="doc.partner_id"
                 t-options='{"widget": "contact", "fields": ["address", "name"], "no_marker": True}' />