CI should respect Nix

[EduardoLR10]

It would be nice to be certain that the current flake.nix provides all that is necessary to build and test the project. This requirement could be tested in CI and any PR that breaks the flake should not be merged without a fix.

[z-silver]

With #12 being merged soon, this can apply just as well to guix.scm. Any PR that breaks the package definition would need to be fixed before being merged.

That said, the current version of guix.scm is actually not quite setup for building the project. I'd need to study how the building machinery of Guix actually works before that can become a thing.