Force empty $TMPDIR inside the chroot

There are some situations (use of libpam-tmpdir) where the $TMPDIR is
defined per user, so any sudo call will define an specific $TMPDIR for
the root user which doesn't exist inside the chroot.

Change-Id: I866651fed520007506c7be83837d3791ed9f2235
Closes-Bug: #1330290
Merge "Force empty $TMPDIR inside the chroot"