stack/erp5: fix software release URL promise that was not executed

Promise from a8271791 (stack/erp5: new promise to check software
release URL of zope instances, 2020-12-04) was never executed. Adding
the section seemed to be change timing to make the upgrade test, but
this was not correct to not run the promise.

For more detail see discussion on
!1414 (comment 189341)