Do not use cached report instance

We must always use a copy of the cached report instance to allow to add
directives to the template before being prepared.

Closes #14460

trytond/trytond/report/report.py

@@ -392,7 +392,7 @@ class Report(URLMixin, PoolBase):
             loader = relatorio.reporting.MIMETemplateLoader()
             klass = loader.factories[loader.get_type(mimetype)]
             template = klass(BytesIO(report.report_content))
-            template = report.set_template_cached(template)
+            report.set_template_cached(template)
         translate = cls._callback_loader(report, template)
         if translate:
             report_context = report_context.copy()