[rubygems/rubygems] Don't write outside of destdir when installing default bundler

https://github.com/rubygems/rubygems/commit/a62d00c5e8

lib/rubygems/commands/setup_command.rb

@@ -418,6 +418,7 @@ By default, this RubyGems will install gem as:
           force: options[:force],
           install_as_default: true,
           bin_dir: bin_dir,
+          install_dir: default_dir,
           wrappers: true
         ).install
       ensure

test/rubygems/test_gem_commands_setup_command.rb

@@ -245,6 +245,8 @@ class TestGemCommandsSetupCommand < Gem::TestCase
   def test_install_default_bundler_gem_with_destdir_flag
     @cmd.extend FileUtils
 
+    FileUtils.chmod "-w", @gemhome
+
     destdir = File.join(@tempdir, 'foo')
     bin_dir = File.join(destdir, 'bin')