Configure ruby's gem paths on NixOS?

[Ivan Brennan]

I'd like to use [interactive_editor](https://github.com/jberkel/interactive_editor), a gem that lets you launch a Vim instance within IRB:

    require 'rubygems'

    require 'interactive_editor'

    vi

I'm trying to set this up on NixOS. My environment.systemPackages includes ruby (2.4.3).

I used bundlerEnv to package interactive_editor and wrote an overlay for it.

I also added the resulting package to my environment.systemPackages, though I'm not sure whether I should, since it's a library and not an executable app.

At the moment, Ruby cannot find the interactive_editor gem in the nix store.

Running `gem environment` shows,

  - GEM PATHS:

     - /home/ivan/.gem/ruby/2.4.0

     - /nix/store/xxxx-ruby-2.4.3/lib/ruby/gems/2.4.0

If I try to require interactive_editor in IRB, I get:

    LoadError: cannot load such file -- interactive_editor

How can I fix this?

An aside: I wanted ruby globally available because the fzf.vim plugin uses a ruby script to generate previews iff the system has a ruby executable.

https://github.com/junegunn/fzf.vim/blob/master/bin/preview.rb

I'm building Vim with ruby support already, so maybe there's another way this could be handled, but I haven't figured it out yet.