WIP: Various nix improvements

[hazelweakly]

Few small things:

• The double single-quote syntax for multi-line strings is much more convenient than here docs and will automatically strip leading spaces correctly.
• outPath is "technically more betterer" than just string interpolating the derivation, but meh.

Otherwise looks nice. Idiomatically, nix code is generally put into a nix directory with only a default.nix and/or a shell.nix file staying at the top level. I'll update the MR with pinning nixpkgs explicitly so that the code will be guaranteed to work regardless of you updating your OS. Especially with stuff like this that might not be touched for a year, you want that :p