[FIX] point_of_sale: Set the correct cashier

After reopening the session in the POS by another user,
the cashier was not correctly identified.

Back-port of this commit: a9caef0ebb

opw:784422

addons/point_of_sale/static/src/js/models.js

@@ -272,6 +272,7 @@ exports.PosModel = Backbone.Model.extend({
                 // replace the current user with its updated version
                 if (user.id === self.user.id) {
                     self.user = user;
+                    self.set_cashier(user);
                 }
             }
             self.users = pos_users;