idf_tools.py: when guessing IDF_PATH, set it in the enviroment
idf_tools.py: when guessing IDF_PATH, set it in the enviroment

This allows the processes launched from idf_tools.py to use the value
of IDF_PATH. One such example is the installation of Python packages,
when requirements.txt uses IDF_PATH to refer to the esp-windows-curses
package.

Closes https://github.com/espressif/esp-idf/issues/4341

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf