Update Sqlpgsql.php

Under a windows environment for a Postgres database, the variable "PGPASSFILE=..." will be interpreted as a command (not found of course, as it's not a valid command).
The result is it asks for password (but silently with the -q parameter), so the drush command is blocked and the password can't be input by the prompt.

In order to set properly the variable, I added a "SET" command if it's under windows platform.
I don't know if it's the best way to handle environment issue here, but it seems to work.