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

tools/idf_tools.py

@@ -1339,6 +1339,7 @@ def main(argv):
         global_idf_path = args.idf_path
     if not global_idf_path:
         global_idf_path = os.path.realpath(os.path.join(os.path.dirname(__file__), ".."))
+    os.environ['IDF_PATH'] = global_idf_path
 
     global global_idf_tools_path
     global_idf_tools_path = os.environ.get('IDF_TOOLS_PATH') or os.path.expanduser(IDF_TOOLS_PATH_DEFAULT)