[FIX] point_of_sale: put pos back in XmlReceipt render context

7836a83b3b was implemented for 9.0 and
uses the pos property. It was forward-ported to saas-11 and up. This
particular property was removed in saas-11 at
467ba386f4 because it was unused.

opw-744222

addons/point_of_sale/static/src/js/screens.js

@@ -1420,6 +1420,7 @@ var ReceiptScreenWidget = ScreenWidget.extend({
     print_xml: function() {
         var env = {
             widget:  this,
+            pos: this.pos,
             receipt: this.pos.get_order().export_for_printing(),
             paymentlines: this.pos.get_order().get_paymentlines()
         };