Added pixbuf cache update to install to properly update loader.cache

loader.cach is being updated to point to the Cellar and not include loaders installed by librsvg. This update fixes that by refreshing the loader cache after install.
I think we probably want to do this like:

```rb
ENV["GDK_PIXBUF_MODULEDIR"] = ...
system "#{Formula["gdk-pixbuf"].opt_bin}/gdk-pixbuf-query-loaders", "--update-cache"
```