do something more reasonable with jobs' log output

[BrianHicks]

Right now, stdout and stderr from jobs get dropped. That's not really tenable! We probably want to store it so we can more easily debug builds.

This might depend on #72 if we decide we want to store it in the database (or if we want to store it in the CAS.)

If someone other than @BrianHicks picks this up, the best bet is probably to have a chat with him on the Roc Zulip about design considerations here!