Revert "[rubygems/rubygems] Use a local temporary directory"

This reverts commit e98455f289047c43a733e61ac6317fb74b68de82.

lib/rubygems/test_case.rb

@@ -302,7 +302,6 @@ class Gem::TestCase < Minitest::Test
     ENV['GEM_VENDOR'] = nil
     ENV['GEMRC'] = nil
     ENV['SOURCE_DATE_EPOCH'] = nil
-    ENV["TMPDIR"] = File.expand_path("tmp")
 
     @current_dir = Dir.pwd
     @fetcher     = nil