nixos again

One of the original goals behind Guix is to make scientific research (or rather computations involved in it) reproducible. You somewhat need to be able to do reproducible packages to achieve that goal. There is built-in `guix challenge' command to build package locally and compare it against the build farms. Heck, there even is `guix time-machine' command that allows you to rewind whole distro to a specific point in time (commit). So yes, Guix takes reproducibility seriously.