It's another attempt to fix chromium builds.
See http://hydra.nixos.org/build/26086977/nixlog/4/raw
Unpacking sources is actually taking more than 2h so build fails.
Instead, rather build it remotely and then copy over the output as
we don't have limits for download time.
See 089bdce621 for reference
cc @aszlig
(cherry picked from commit cef54e7d67)
Signed-off-by: Domen Kožar <domen@dev.si>
94e3e1195e