--- a/tool/rbinstall.rb.~1~	2020-12-25 04:33:01.000000000 +0100
+++ b/tool/rbinstall.rb	2020-12-25 10:05:34.629096405 +0100
@@ -897,7 +897,7 @@
 end
 
 def install_default_gem(dir, srcdir, bindir)
-  gem_dir = Gem.default_dir
+  gem_dir = ENV['GEM_DESTDIR']
   install_dir = with_destdir(gem_dir)
   prepare "default gems from #{dir}", gem_dir
   makedirs(Gem.ensure_default_gem_subdirectories(install_dir, $dir_mode).map {|d| File.join(gem_dir, d)})
@@ -1018,7 +1018,7 @@
   end
   Gem.instance_variable_set(:@ruby, ruby_path) if Gem.ruby != ruby_path
 
-  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