Merge pull request #9616 from ruby/claude/great-archimedes-71fdb7

Fix ruby-core tarball detection in linked git worktrees

Author: hsbt

spec/support/path.rb

@@ -357,7 +357,9 @@ def man_tracked_files_glob
     end
 
     def ruby_core_tarball?
-      !git_root.join(".git").directory?
+      # A tarball checkout has no `.git` entry at all. Note that `.git` may be
+      # a file rather than a directory in linked git worktrees.
+      !git_root.join(".git").exist?
     end
 
     def rubocop_gemfile_basename