reset of PYTHONHOME as early as possible

[matysek]

Move the reset of PYTHONHOME as early as possible, that is immediately after Py_Initialize. Ideally, there should be some code like:

   save PYTHOHOME current value
   set PYTHONHOME
   Py_Initialize()
   reset PYTHONHOME to original value

that immediately makes clear where PYTHONHOME is useful.

I just checked the code in launch.c and PYTHONHOME is set immediately before Py_Initialize(). So we just need to reset it back to the original value a few lines after.