torture: Skip vmlinux check for kvm-again.sh runs

The kvm-again.sh script reruns an previously built set of kernels, so
the vmlinux files are associated with that previous run, not this on.
This results in kvm-find_errors.sh reporting spurious failed-build errors.
This commit therefore omits the vmlinux check for kvm-again.sh runs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>