Wrong order or lower layers when building overlays for OEM and third-party drivers

When doing an offline install, ubuntu-drivers would sometimes list a package that is available in the archive but not present in the pool. This would lead to a failed install when trying to download/install such package.

This is not something we would expect since we run apt-get update (with only the pool configured when offline) in the install tree.

However, it turned out that we create the overlay with the lower layers specified in the wrong order - which essentially makes APT indexes visible in the source tree also visible in the OEM/third-party driver overlay.

When calling setup_overlay(lowers=[a, b, c]), Subiquity invokes mount with lowerdir=c:b:a (in the reverse order).

This means that c is top, b is middle and a is bottom.

For the OEM and third-party drivers, we build overlays that are based on:

    the source tree
    the configured tree
    the install tree

Unfortunately, we are stacking the lower layers in the wrong order.