It would be good to avoid unnecessary calls to 'substitute-binary' when the needed items are already in the store, e.g.:
--8<---------------cut here---------------start------------->8--- mhw@jojen:~$ guix build qt substitute-binary: updating list of substitutes from 'http://hydra.gnu.org'... /gnu/store/1si4b0cww9xzf23i6krkvr2h3vw67jgz-qt-5.4.1 --8<---------------cut here---------------end--------------->8--- Mark
