runtime: Search gconv in the correct dir when it starts with /app

When we search for gconv in the provider, if the directory we derived from libc starts with "/app/", we should not prepend "/usr/" because we don't expect to have "/usr/app/" in the provider.

Without this patch, pressure-vessel in a Flatpak container fails to load i386 gconv with the following message: "We were expecting the gconv modules directory in the provider to be located in "/app/lib/i386-linux-gnu/gconv", but instead it is missing"