[lit] Respect PYTHONPATH

If a user has PYTHONPATH set in the environment, append new entries to
it rather than blindly setting PYTHONPATH to a fixed string. This
allows tests to, for example, find psutil if it is in
PYTHONPATH. Without this change, lit will detect psutil but then
various tests will fail because PYTHONPATH has been overwritten and
psutil cannot be found.

llvm-svn: 350536