cmake: Fix for Python files executed directly, not via PYTHON variable

cmake: Fix for Python files executed directly, not via PYTHON variable

A problem if the Python interpreter used for idf.py (or set via PYTHON
variable) didn't match
"/usr/bin/env python" (or the associated executable for .py files, on
Windows).

Closes https://github.com/espressif/esp-idf/issues/3160
Possibly also fix for https://github.com/espressif/esp-idf/issues/2936

Adds build system test to catch any future direct execution of Python in
the standard build process.