Track upstream `nixpkgs`, not `profianinc/nixpkgs`

[rvolosatovs]

The only thing really necessary for us here is the Enarx kernel and Benefice NixOS module.

We should:

• [ ] move the Benefice module to https://github.com/profianinc/benefice
• [ ] track recently-added https://github.com/enarx/linux flake (blocked by https://github.com/enarx/linux/issues/5)