Fail fast when running on forks

Forks of this Allen aren't able to run the tests correctly due to the use of custom runners. This change makes the pipeline fail early with a clear message rather than getting stuck waiting for a suitable runner.

A example failing pipeline that ran against my fork can be seen at https://gitlab.cern.ch/cburr/Allen/pipelines/1704953

Follow up to: !396 (comment 3527498)