It just makes things unnecessarily complicated. We can just exit
without cleaning anything up, since the only thing to do is unmark
builds and build steps as busy. But we can do that by having systemd
call "hydra-queue-runner --unlock" from ExecStopPost.
If multiple threads create a step for the same build, they could get
the same "max(stepnr)" and allocate conflicting new step numbers. So
lock the BuildSteps table while doing this. We could use a different
isolation level, but this is easier.
This removes the need for Nix's build-remote.pl.
Build logs are now written to $HYDRA_DATA/build-logs because
hydra-queue-runner doesn't have write permission to /nix/var/log.
