prebuild_toplevel awaited nix build with a bare child.wait().await, so
a wedged nix-daemon (unreachable remote builder, stuck build slot)
hung the rebuild job forever with no way for the job queue to recover
short of restarting hive-c0re.
Mirrors #4741's hive-priv fix: the child now leads its own process
group, and after PREBUILD_TIMEOUT (1h, four times CI's observed
cold-cache flake check) the whole group is SIGKILLed and the call
fails with a named timeout error instead of hanging.
Refs #4723, #4741
5409f8160b