gs-plugin-job: Only report the timeout when it is actually enforced

Most actions do not enforce a timeout. Reporting a timeout when it is not enforced is confusing, leading to reports that a certain action started with a certain timeout when that is not true or that it completed well-passed its timeout.

I wonder if this enforcement should also happen in gs_plugin_job_set_timeout to avoid plugins erroneously thinking that a timeout can be set, when that is not the case. Also, might it be worth documenting? It would have helped with developing tests for an out-of-tree plugin