PythonInstalled: set PYTHONPATH, not prepend

An issue could arise when brewing a formula that
has Python 2.x and 3.x support and 2.x is the system Python
but 3.x is a brewed one. The idea about prepending was that
user configured PYTHONPATH could be used in formulae.
Now, instead and if needed, inside the `python do` block
one can still append to PYTHONPATH.