Fix macOS ARM64 PYTHONPATH in setup script

Fix PYTHONPATH variable expansion issue in macOS setup script generator.

Problem:
- PYTHONPATH was showing as "/lib/python:" instead of full path
- Variable ${USD_INSTALL_DIR} was being expanded during script generation
- At generation time, the variable doesn't exist, so it expanded to empty

Solution:
- Change heredoc from << EOF to << 'EOF' (with single quotes)
- This prevents variable expansion during generation
- Variables now expand at runtime when script is sourced

This fixes the Python import error:
  ModuleNotFoundError: No module named 'pxr'

Now PYTHONPATH will correctly be:
  ${USD_INSTALL_DIR}/lib/python (expands at runtime)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: syoyo

05-setup-env-minsizerel-macos.sh

@@ -60,7 +60,7 @@ export DYLD_LIBRARY_PATH="${USD_INSTALL_DIR}/lib:${TBB_DIR}/lib:${DYLD_LIBRARY_P
 EOF
 
 # Add Python version-specific path
-cat >> "${SETUP_SCRIPT}" << EOF
+cat >> "${SETUP_SCRIPT}" << 'EOF'
 export PYTHONPATH="${USD_INSTALL_DIR}/lib/python:${PYTHONPATH}"
 EOF