[ruby/rubygems] Address review feedback on tmp directory path resolution

Use source_root.parent instead of hardcoded relative path to derive the
repository root, so the code remains correct if the support file is ever
moved. Also fix the example Gemfile source path in DEBUGGING.md to
reflect the new top-level tmp location.

ruby/rubygems@b1b039817e

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

spec/bundler/support/path.rb

@@ -123,7 +123,7 @@ def tmp_root
         end
         Pathname(real)
       else
-        Pathname.new("../..").expand_path(__dir__).join("tmp")
+        (ruby_core? ? source_root : source_root.parent).join("tmp")
       end
     end