I added `asciidoctor` to the nativeBuildInputs as I saw warnings that it
wasn't found as I built the tool.
I also stopped using `rec` syntax as the nixpkgs manual recommends
[against it](https://nixos.org/manual/nixpkgs/unstable/#mkderivation-recursive-attributes).
However, `rustPlatform.buildRustPackage` doesn't allow the use of
`mkDerivation (finalAttrs: {` form, so it's just a simple `let` binding
here.
