Run a task only once within jinja2 loop

As a follow up to this question we actually tried broadcasting to set the run mode to skip in a post-script to the task. This caused the task to be marked as “failed” and the workflow to stop. I think here we ran into the polling issue that was discussed in @sparonuz’s post https://cylc.discourse.group/t/cylc-broadcast-error-workflowstopped/. So this approach is definitively not an option