wrap_tails_create_iuks: let tails-create-iuk manage the lock file

If we create the lock file as the non-root user in the wrapper,
then tails-create-iuk fails with:

  flock: cannot open lock file /tmp/mksquashfs-636598.lock: Permission denied
  "sudo" unexpectedly returned exit value 66 at /path/to/tails/config/chroot_local-includes/usr/src/iuk/bin/../lib/Tails/IUK.pm line 435.

I believe that's because I have the fs.protected_regular sysctl set to 2.

Now, as kibi explained in 771db0d8, this implies
we cannot delete the lock file in the wrapper as the non-root user, so let's
use sudo to do that: the wrapper already assumes sudo anyway.