apparmor: add support for mkosi integration working directory

mkosi integrates with swtpm to automatically set up and build
VMs with vTPM support. The working directory is in an ephemeral
namespace that appears as /work/tmp/, and apparmor stops swtpm
from creating the local state files (lockfile, etc).
Add a policy entry to allow this to work.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>