Avoid deleting previous PYTHONPATH

With the previous putenv command, if an user had already set the PYTHONPATH
environment variable, its value was deleted and substituted by os.getcwd(),
which may prevent the installation if the dependencies are installed in a
different path. Now, it appends this value to the previous value set in the
sys.path, so the user configuration is kept.

Reviewed at https://reviews.reviewboard.org/r/6001/