Allow Ponos tasks to list and download artifacts from any task that the process creator has guest access to

Refs https://redmine.teklia.com/issues/3653#change-27167

After a task generates a dataset's generic artifacts, other tasks need to be able to download those artifacts to run some worker- or model-specific preprocessing or train a model. Since multiple processes might run on the same datasets after their artifacts are built, tasks from other processes will need to be able to access those artifacts.

Regular users can already list artifacts and download them from any process they have a guest access to using ListArtifacts and DownloadArtifact, so allowing Ponos task authentication to use the access rights of the process' creator would unlock this.