bug#74204: Guix is not reproducible

[Jakob Kirsch]

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.