Add nix build files

[Multipixelone]

Add build files for the nix package manager.

This lets the script be run with something like: nix run github:obskyr/khinsider

I know you want to keep the repo super simple so If you aren’t looking for additions like this, no worries! I just figured since I wrote it for myself I’d push it upstream. Been using your script for years now, thanks for it! :-)