Revert "[ruby/rubygems] Revert "Merge pull request #8989 from nobu/test-tmpdir""

This reverts commit 6e00da098abacd8c9ef0ea621cb1832e654b1813.

Author: hsbt

bundler/spec/support/path.rb

@@ -114,7 +114,7 @@ def tmp(*path)
     end
 
     def tmp_root
-      source_root.join("tmp")
+      ruby_core? && (tmpdir = ENV["TMPDIR"]) ? Pathname(tmpdir) : source_root.join("tmp")
     end
 
     # Bump this version whenever you make a breaking change to the spec setup

bundler/spec/support/rubygems_ext.rb

@@ -43,7 +43,7 @@ def test_setup
       # sign extension bundles on macOS, to avoid trying to find the specified key
       # from the fake $HOME/Library/Keychains directory.
       ENV.delete "RUBY_CODESIGN"
-      ENV["TMPDIR"] = Path.tmpdir.to_s
+      ENV["TMPDIR"] = Path.tmpdir.to_s unless Path.ruby_core?
 
       require "rubygems/user_interaction"
       Gem::DefaultUserInteraction.ui = Gem::SilentUI.new