restore: don't race when closing cg yard

TASK_HELPERs are created with CLONE_FILES, so if we always close the cg yard
here, it will close it for the other helpers and cause problems. Instead, we
close it much later, in code only called by alive tasks, to ensure that there
is no conflict.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>