interpreters/python: Enable using pip to install Python packages

This commit enables using `pip` as a pre-compiled (pyc) built-in
distributed along with cpython.

Signed-off-by: Tiago Medicci <tiago.medicci@espressif.com>

interpreters/python/python_wrapper.c

@@ -198,5 +198,7 @@ int main(int argc, FAR char *argv[])
 
   setenv("PYTHON_BASIC_REPL", "1", 1);
 
+  setenv("PYTHONPATH", CONFIG_INTERPRETERS_CPYTHON_PYTHONPATH, 1);
+
   return py_bytesmain(argc, argv);
 }