fix environment handling of bld_utils.py

  - system environment has the highest priority now
  - convert PATH to Path on windows in every cases where we
    change the PATH value

Change-Id: Ie46e280e066b6ff856c0abe4eb770f83c1c8093e
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>