--- a/tool/rbinstall.rb	2022-11-13 08:12:27.373041649 +0100
+++ b/tool/rbinstall.rb	2022-11-13 08:16:09.110406705 +0100
@@ -994,7 +994,7 @@
 end
 
 install?(:ext, :comm, :gem, :'bundled-gems') do
-  gem_dir = Gem.default_dir
+  gem_dir = ENV['GEM_DESTDIR']
   install_dir = with_destdir(gem_dir)
   prepare "bundled gems", gem_dir
   RbInstall.no_write do