Fix: Skip system temp directories in rootdir detection (issue #13822)

[AddyM]

Summary

Fixes #13822

When /tmp/setup.py exists on the system, pytest incorrectly identifies /tmp as the project root, causing test node IDs to include full paths under /tmp instead of relative paths.

Changes

• Added _is_system_temp_or_parent() helper function to detect system temp directories
• Modified determine_setup() to skip system temp directories when searching for setup.py
• Uses tempfile.gettempdir() for cross-platform compatibility

Testing

• Verified helper function correctly identifies system temp directory
• Tested that regular directories are not flagged as temp
• Manual testing confirms /tmp/setup.py no longer affects rootdir detection

Backward Compatibility

• No breaking changes
• Users can still use --rootdir to override if needed
• Only affects systems with setup.py in system temp directories