apparmor: Use more generic allow rule for pivot

Recent fixes in the apparmor kernel code is now making at least the CI
environment and quite possibly some others fail due to an invalid path
in the pivot_root stanza.

So update both lines to allow a more generic pivot_root call for
anything in LXC's work directory.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>