flatpakref install doesn't search for compatible arches

[alexlarsson]

If you're on say x86-64 and a flatpakref points to a repo that only has an i386 version, then this works:

$ flatpak install --arch=i386  --user https://dl.flathub.org/build-repo/1646/com.xnview.XnRetro.flatpakref

But this fails:

$ flatpak install  --user https://dl.flathub.org/build-repo/1646/com.xnview.XnRetro.flatpakref
error: No such ref 'app/com.xnview.XnRetro/x86_64/stable' in remote xnretro-origin

We should make it look for the fallback arches too.