The default cache location in gdk-pixbuf has to be defined.  The default location is usually `libdir/gdk-pixbuf-2.0/2.10.0/loaders.cache` per https://developer.gnome.org/gdk-pixbuf/unstable/gdk-pixbuf-query-loaders.html.  The dependent programs won't run if the environment variable isn't set.

The glob is for the `2.10.0` since this version can change with an update.  My thinking is that  other formulas that depend on gdk-pixbuf will also need to make this post install call, and I'm aiming for consistency.

I know you recommended writing to `var`, but the default location is in the `lib` dir.  It shouldn't matter of course, but I wanted to make it as obvious as possible should anyone have any future issues.