necessary that at least one build hook doesn't return "postpone",
otherwise nix-store will barf ("waiting for a build slot, yet there
are no running children"). So inform the build hook when this is
the case, so that it can start a build even when that would exceed
the maximum load on a machine.
list like
root@example.org x86_64-linux /root/.ssh/id_buildfarm 1
root@example.org i686-darwin /root/.ssh/id_buildfarm 1
This is possible when the Nix installation on example.org itself has
remote builds enabled.