I've recently noticed that `guix challenge guix` fails since guix isn't reproducible at the moment. As someone pointed out on XMPP, this is due to parallelism issues with Guile so running `guix build guix --no-grafts --cores=1 --check` reliably produces the same output.
I've sent a simple patch in #74112 to address this and I think this is important since the main guix package should definitely be reproducible so it can be trusted.
