With GTK_USE_PORTAL=1, try to use the portal even if we if we use
unsupported features. With GTK_USE_PORTAL=0, never use the portal,
as before. What we really want here is to detect the situation
of "sandboxed without full $HOME access" and use the portal in
those cases. If we make the portal smart enough to hand back
direct uris for files that the app can access, we may simplify
the check to just "sandboxed".