Reduced substitute-binary latency

[Ludovic Courtès]

Commit 50dc489 does the obvious thing to reduce latency when looking for
substitutes on hydra.gnu.org.

Thus, something like:

--8<---------------cut here---------------start------------->8---
$ sudo rm /nix/var/nix/substitute-binary/cache/*

$ time ./pre-inst-env guix build coreutils -n
The following files would be downloaded:
   /nix/store/bm7rzxnmlgprx0i5jfmpwvrc5h5cq6y4-coreutils-8.21
   /nix/store/xwaywgm456l9f3skjsviphrp6dc9h002-coreutils-8.21-debug
--8<---------------cut here---------------end--------------->8---

goes from 11.8s to 3.7s on my machine, with networking latency
artificially increased with

  sudo tc qdisc add dev eth0 root netem delay 1500ms

Before that there’d be at most one thread per core, which was silly.

Ludo’.