Allow fetching WorkerRuns on one's own local process

While testing !1964 (merged), I found that I could not access the details of the re-imported WorkerRuns. They are on a Local process, which does not have a corpus, and thus RetrieveWorkerRun just excludes it and returns HTTP 404.

We should allow retrieving the WorkerRun of one's own local process, and maybe also allow retrieving the WorkerRun of the local processes of other users. This is likely related to changes in WorkerRun rights in #1415.