Gem::Commands::RebuildCommand#prep_dirs

# File tmp/rubies/ruby-3.4.1/lib/rubygems/commands/rebuild_command.rb, line 166
def prep_dirs
  rebuild_dir = Dir.mktmpdir("gem_rebuild")
  old_dir = File.join(rebuild_dir, "old")
  new_dir = File.join(rebuild_dir, "new")

  FileUtils.mkdir_p(old_dir)
  FileUtils.mkdir_p(new_dir)

  [old_dir, new_dir]
end