fix(sdk-lib): Mount a fresh /run directory.

There is no need to arbitrarily bind mount all of the host system's /run
into the chroot. In fact this causes issues when the host system's /run
isn't set up in a way this script anticipates. Namely the user runtime
directory in /run/usr/$UID is another tmpfs mount on my system, leaving
the underlying directory node that is bind-mounted in with the wrong
ownership. Behave a little more like a responsible container and use a
fresh /run but continue binding /run/shm for whatever versions of Ubuntu
that depended on that behavior. Not strictly needed but go ahead and
create the user runtime directory with the correct permissions.