[ruby/rubygems] Add a Mutex to prevent a bug on Ruby 3.2:

- There are failing tests on Ruby 3.2 due to an expecation that
  checks wheter stderr is empty. In this case, stderr outputs a
  warning from Ruby "warning: loading in progress, circular require
  considered harmful", but this is not true.

  This is a bug in Ruby 3.2 that I also encountered in ruby/rubygems#9100 (comment)
  The problem is that Ruby 3.2 wrongly output this warning when
  multiple threads try to require a file at the same time.
  See https://bugs.ruby-lang.org/issues/19415 which was fixed in Ruby
  3.2.2.

  I opted to add a Mutex, as otherwise users on Ruby 3.2 will have this
  warning output.

ruby/rubygems@713dd5c0e1

Author: Edouard-chin

lib/bundler/source/rubygems.rb

@@ -9,6 +9,7 @@ class Rubygems < Source
 
       # Ask for X gems per API request
       API_REQUEST_SIZE = 100
+      REQUIRE_MUTEX = Mutex.new
 
       attr_accessor :remotes
 
@@ -521,7 +522,7 @@ def rubygems_gem_installer(spec, options)
           path = fetch_gem_if_possible(spec, options[:previous_spec])
           raise GemNotFound, "Could not find #{spec.file_name} for installation" unless path
 
-          require_relative "../rubygems_gem_installer"
+          REQUIRE_MUTEX.synchronize { require_relative "../rubygems_gem_installer" }
 
           installer = Bundler::RubyGemsGemInstaller.at(
             path,