Would any clever **ruby** devs here know how to force `bundlerEnv` to use a patched version of `bundler` (with `nix-shell`)?

After studying its [source](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/ruby-modules/bundled-common/default.nix), I thought it would be enough to to import `bundlerEnv` like this: 
`callPackage <nixpkgs/pkgs/development/ruby-modules/bundler-env> { bundler = myBundlerEnv; };`

I can see that my patched version of `bundler` does get built (in `/nix/store`), but that isn't the `bundler` derivation that actually gets used (as confirmed by running `cat $(which ruby)` inside the `nix-shell`)

🤔