Smaller CI intermediate tarballs (option 2)

[Googulator]

This is a different option for making CI tarballs smaller: we revert to using a small seed tarball, plus packages built in each pass. The seed tarball is built as early as possible (i.e. right after we have bash), everything beyond that is passed in the form of repo packages.

To make this work properly, in each pass, rootfs.py needs to be run once for each previous pass to preseed from, and once more for the current pass.

This option depends on #368.