Fix problem when downgrading rubygems

Take the following scenario:

* You have a rubygems version with the new plugins system.
* You run `gem install yard`, so that `<gem_home>/plugins/yard_plugin.rb` is generared.
* You downgrade rubygems to a version with the old plugin system, say, 3.1.2.
* You uninstall yard.
* You upgrade rubygems again.

In this case, the final upgrade will fail because rubygems will try to
require a `yard` rubygems plugin to points to a file that does not exist
because the `yard` gem is not installed.

This change fixes that problem.

lib/rubygems/commands/update_command.rb

@@ -170,7 +170,16 @@ def install_rubygems(version) # :nodoc:
     Dir.chdir update_dir do
       say "Installing RubyGems #{version}"
 
-      installed = system Gem.ruby, '--disable-gems', 'setup.rb', *args
+      if Gem::Version.new(version) < Gem::Version.new("3.2.a")
+        require "tmpdir"
+        tmpdir = Dir.mktmpdir
+        FileUtils.mv Gem.plugins_dir, tmpdir
+        installed = system Gem.ruby, '--disable-gems', 'setup.rb', *args
+        FileUtils.mv File.join(tmpdir, "plugins"), Gem.plugins_dir unless installed
+      else
+        installed = system Gem.ruby, '--disable-gems', 'setup.rb', *args
+      end
+
       say "RubyGems system software updated" if installed
     end
   end