`nerdctl kill` shouldn't set `containerd.io/restart.explicitly-stopped` label for non-stop signals

https://github.com/containerd/nerdctl/blob/592c011fc97e98c119637b6af0f6fedfd2b98bdb/pkg/cmd/container/kill.go#L82-L127

nerdctl kill can be used for non-stop signals such as HUP, so it shouldn't unconditionally set containerd.io/restart.explicitly-stopped label