Prior to this it doesn't seem to be possible to customize the version of
bundler used in a bundlerEnv app. This change allows something like the
following:
```nix
let bundler = pkgs.buildRubyGem rec {
gemName="bundler";
name="bundler-2.2.11";
version="2.2.11";
source.sha256="1izx6wsjdm6mnbxazgz1z5qbhwrrisbq0np2nmx4ij6lrqjy18jf";
};
in pkgs.bundlerEnv.override { inherit bundler; } {
name="test";
gemdir=./.;
}
```
to use bundler 2.2.11 rather than the 2.1.5 default.
878bae1142