ruby: replace bundled RubyGems with our RubyGems

This should prevent problems caused by trying to install our own
RubyGems over the top of the one that comes with Ruby.

pkgs/development/interpreters/ruby/default.nix

···
           }).${ver.majMinTiny};
 
         postUnpack = opString rubygemsSupport ''
-          cp -r ${rubygems} $sourceRoot/rubygems
+          rm -rf $sourceRoot/{lib,test}/rubygems*
+          cp -r ${rubygems}/lib/rubygems* $sourceRoot/lib
+          cp -r ${rubygems}/test/rubygems $sourceRoot/test
         '';
 
         postPatch = if atLeast25 then ''
···
           EOF
 
           rbConfig=$(find $out/lib/ruby -name rbconfig.rb)
-        '' + opString rubygemsSupport ''
-          # Update rubygems
-          pushd rubygems
-          chmod +w bundler/bundler.gemspec
-          ${buildRuby} setup.rb
-          popd
         '' + opString docSupport ''
           # Prevent the docs from being included in the closure
           sed -i "s|\$(DESTDIR)$devdoc|\$(datarootdir)/\$(RI_BASE_NAME)|" $rbConfig