Add an option to disable writing build logs

[edolstra]

On Hydra build machines, it's pointless to write build logs to /nix/var/log/nix/drvs, since they're also kept on the main Hydra server. And since logs are not garbage-collected, they will eventually fill up the disk. So it would be nice to have an option to disable writing logs.

[CMCDragonkai]

@edolstra Is there any way to garbage collect the build logs? I've searched through all the manuals.