--no-install-recommends is a big help, whether we're installing all of
`erlang` or just `erlang-wx`, but `erlang-wx` is still so close in size
that we still just use `erlang`.
A bit fiddly, but this lets us run a realistic erlang install script
from userspace, and then re-enter userspace later without wiping the
installation.
Now it extracts and calls the program properly, if it isn't already
installed.
Also, the script `debian/cleanup_everything` removes debootstrap and
both of the created Debian roots, returning the repository to the same
state it clones in as.
