I've recently noticed that `guix challenge guix` returns different hashes for each substitute server and also every build with `guix build --no-grafts guix` fails.
Running `guix build --no-grafts guix --cores=1` seems to reliably produce the same hash though. As someone pointed out on XMPP, Guile seems to have issues with parallel builds so this patch disables them for the guix package.

I think this has high importance because the main guix package should be reproducible in order to trust the whole chain of packages.