[FIX] account_check_printing: error during migration

During the conversion of the server action "action_account_print_checks",
the "self.env" variable was wrongly kept. Now in server actions we
are using the variable "env" directly.

addons/account_check_printing/data/account_check_printing_data.xml

@@ -16,7 +16,7 @@
             <field name="model_id" ref="account.model_account_payment"/>
             <field name="code">
                 if context.get('active_model') == 'account.payment' and context.get('active_ids'):
-                    action = self.env['account.payment'].browse(context['active_ids']).print_checks()
+                    action = env['account.payment'].browse(context['active_ids']).print_checks()
             </field>
         </record>