tests/docker: fix make-archive-maybe

make-archive-maybe expects an archive path relative
to $1, but receives a path relative to the current directory.  Redirect
the output outside the subshell to bypass the "cd $1".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1465224417-141321-2-git-send-email-pbonzini@redhat.com
Signed-off-by: Fam Zheng <famz@redhat.com>
