[Nix-dev] can't override rubygems version

[Brandon Dimcheff]

Hi there,

I have an old project that needs an old rubygems version, so I'm trying
to override it in a nix expression with overrideDerivation like so:

https://github.com/bdimcheff/oldgems.nix/blob/master/default.nix

Unfortunately that doesn't work, and I end up with the version in
nixpkgs instead, despite it downloading the appropriate version.  I have
a feeling I'm missing something silly, but the rubygems package is
implemented differently than most others, so maybe that's a problem?
https://github.com/NixOS/nixpkgs/blob/37dad0ee0b36dc6964ac87667f2c7c3723b22dd1/pkgs/top-level/all-packages.nix#L5109-L5112

Anyone have any suggestions?

Thanks,
Brandon