BaseTools/PosixLike: honor pre-set PYTHONPATH

BaseTools/PosixLike: honor pre-set PYTHONPATH

Utilities written in Python may depend on external (preinstalled) Python
packages; for example, Ecc depends on "antlr_python_runtime-3.0.1". Such
packages need not be installed system-wide, as long as they are reachable
through PYTHONPATH. Therefore we shouldn't overwrite the user's PYTHONPATH
with "BaseTools/Source/Python"; instead, we should prepend the latter to
the former.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=896
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>