Fix proper pygobject-codegen-2.0 path.

Currently running `pygtk-codegen-2.0` fails because it can't find their deprecation alternative `pygobject-codegen-2.0` in their bin path(e.g. /usr/local/Cellar/pygtk/2.24.0/bin).

My PR handles this issue by setting their search path in homebrew bin path.

Related Issue: #28407, Installation of vte fails.