HPLIP's getSystemPPDs() function relies on searching for PPDs below common FHS
paths. None of these exist on NixOS, but the code assumes that at least one of
the directories will be found, and crashes when it doesn't (cups_ppd_path is
None and the code passes that to os.path.join).
A usable PPD search path for the running system on NixOS is
/var/lib/cups/path/share, so this patches the source to check this path as well.
This should fix the NixOS case and keep non-NixOS cases working too.
