buildRubyGem: don't set the GEM_HOME

bundler for example needs to have the GEM_HOME being passed trough to
function properly.

For gems that are loading content dynamically, or can use plugins, use
buildRubyGem.

For executables that are wrapped in their own sealed thing use
bundlerEnv.

pkgs/development/ruby-modules/gem/gem-post-build.rb

@@ -7,7 +7,7 @@ ruby = File.join(ENV["ruby"], "bin", RbConfig::CONFIG['ruby_install_name'])
 out = ENV["out"]
 bin_path = File.join(ENV["out"], "bin")
 gem_home = ENV["GEM_HOME"]
-gem_path = ENV["GEM_PATH"].split(":")
+gem_path = ENV["GEM_PATH"].split(File::PATH_SEPARATOR)
 install_path = Dir.glob("#{gem_home}/gems/*").first
 gemspec_path = ARGV[0]
 
@@ -64,11 +64,16 @@ spec.executables.each do |exe|
 # this file is here to facilitate running it.
 #
 
-Gem.use_paths "#{gem_home}", #{gem_path.to_s}
-
 require 'rubygems'
 
-load Gem.bin_path(#{spec.name.inspect}, #{exe.inspect})
+Gem.paths = {
+  'GEM_PATH' => (
+    ENV['GEM_PATH'].to_s.split(File::PATH_SEPARATOR) +
+    #{([gem_home] + gem_path).to_s}
+  ).join(File::PATH_SEPARATOR)
+}
+
+load Gem.activate_bin_path(#{spec.name.inspect}, #{exe.inspect}, #{spec.version.to_s.inspect})
     EOF
   end