Add skyflake, notnft and nixos-router flakes

[spacekitteh]

No description provided.

[RaitoBezarius]

Kirelagin is unmaintained, I am not sure it's a good idea.

[spacekitteh]

Alright, I'll remove that one.

[raboof]

nixos-router seems to fail with

             error: attribute 'utils' missing
             at /nix/store/p08jvcw823dpbc6rf5d65axaf4akzj4f-source/lib/modules.nix:554:28:
                553|         addErrorContext (context name)
                554|           (args.${name} or config._module.args.${name})
                   |                            ^
                555|       ) (functionArgs f);

[raboof]

I'll close this for now, but feel free to follow up