[nix] Add receipts for building with nix

[ein-shved]

Not sure, if you happy to see this receipts in mainstream, but with NIX the build instruction degenerates to one simple line:

nix build github:dheijl/swyh-rs

It is reliable and portable, you do not even need to clone the repo.

If you never used NIX you can try it on my fork of your project:

apt install nix
nix --extra-experimental-features 'flakes nix-command' run github:ein-shved/swyh-rs/nix

[dheijl]

Thanks, I'll try it out!