When an artifact is requested from hydra the output is first copied
from the nix store into memory and then sent as a response, delaying
the download and taking up significant amounts of memory.
As reported in https://github.com/NixOS/hydra/issues/1357
Instead of calling a command and blocking while reading in the entire
output, this adds read_into_socket(). the function takes a
command, starting a subprocess with that command, returning a file
descriptor attached to stdout.
This file descriptor is then by responsebuilder of Catalyst to steam
the output directly
(cherry picked from commit 459aa0a5983a0bd546399c08231468d6e9282f54)
5cb82812f2