Hi guix!

This patch allows to count builds and downloads separately. The idea is
that downloads need bandwidth, but no CPU, while builds do not need
bandwidth, but need CPU. With this patch, guix will be able to download
substitutes while building unrelated packages. Currently, guix needs to
wait for the download to finish before proceeding to the build. This
should reduce the time of guix commands that need to build and download
things at the same time.

What do you think?