bundlerEnv: Allow overriding bundler

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.

pkgs/development/ruby-modules/bundler-env/default.nix

···
 let
   inherit (import ../bundled-common/functions.nix {inherit lib ruby gemConfig groups; }) genStubsScript;
 
-  basicEnv = (callPackage ../bundled-common {}) (args // { inherit pname name; mainGemName = pname; });
+  basicEnv = (callPackage ../bundled-common { inherit bundler; }) (args // { inherit pname name; mainGemName = pname; });
 
   inherit (basicEnv) envPaths;
   # Idea here is a mkDerivation that gen-bin-stubs new stubs "as specified" -