nixbot: Use NixOS module for stuff that needs one-time setup

[infinisil]

There's some things that require a one-time setup. It would be possible to embed that logic into the executable, but that would be bothersome and error-prone. Much simpler is to use a NixOS module to generate these things and pass the path to the executable.

This includes:

• The nixpkgs checkout
• Nix files which I currently manually insert (allowing fetching from the web from the shell would solve that too)
• The nix-index cache (this one might be tricky)
• The index file containing which nixpkgs file is committed to how many times (used for ,find, currently done manually too..)

Could also add multiple channels like this rather easily.

So the NixOS module should take options to configure everything, and output a running service containing all it needs to run successfully.