ruby: make buildInputs work

pkgs/development/interpreters/ruby/bundler-env.nix

···
   );
 
   needsPreInstall = attrs:
-    (attrs ? preInstall) || (attrs ? buildInputs);
+    (attrs ? preInstall) || (attrs ? buildInputs) || (attrs ? nativeBuildInputs);
 
   createPreInstallers = lib.fold (next: acc:
     if !needsPreInstall next
···
     else acc + ''
       cp ${writeScript "${next.name}-pre-install" ''
         #!/bin/sh
+
         buildInputs="${toString (next.buildInputs or [])}"
+        nativeBuildInputs="${toString (next.nativeBuildInputs or [])}"
+
         source ${stdenv}/setup
 
         ${next.preInstall or ""}
 
-        ${ruby}/bin/ruby -e 'ENV.inspect' > env/${next.name}
+        ${ruby}/bin/ruby -e 'print ENV.inspect' > env/${next.name}
       ''} pre-installers/${next.name}
     ''
   ) "" (attrValues instantiated);

pkgs/development/interpreters/ruby/bundler-head.nix

···
   src = fetchgit {
     url = "https://github.com/bundler/bundler.git";
     rev = "a2343c9eabf5403d8ffcbca4dea33d18a60fc157";
-    sha256 = "1f0isjrn4rwak3q6sbs6v6gqhwln32gv2dbd98r902nkg9i7y5i0";
+    sha256 = "1l4r55n1wzr817l225l6pm97li1mxg9icd8s51cpfihh91nkdz68";
     leaveDotGit = true;
   };
   dontPatchShebangs = true;

pkgs/development/interpreters/ruby/monkey_patches.rb

···
 require 'bundler'
 
+# Undo the RUBYOPT trickery.
+opt = ENV['RUBYOPT'].dup
+opt.gsub!(/-rmonkey_patches.rb -I [^ ]*/, '')
+ENV['RUBYOPT'] = opt
+
 Bundler.module_eval do
   class << self
     # mappings from original uris to store paths.
···
     # swap out ENV
     def nix_with_env(env, &block)
       if env
+        old_env = ENV.to_hash
         begin
-          old_env = ENV.to_h
           ENV.replace(env)
           block.call
         ensure
-          ENV.replace old_env
+          ENV.replace(old_env)
         end
       else
         block.call
···
     pre_installer = "pre-installers/#{spec.name}"
     if File.exist?(pre_installer)
       system(pre_installer)
+      unless $?.success?
+        Bundler.ui.error "The pre-installer script for #{spec.name} failed!"
+        exit 1
+      end
       env = eval(Bundler.read_file("env/#{spec.name}"))
+      unless env
+        Bundler.ui.error "The environment variables for #{spec.name} could not be loaded!"
+        exit 1
+      end
       Bundler.nix_with_env(env) do
         original_install_gem_from_spec(spec, standalone, worker)
       end