[velero] fix missing deployment selector

[jfcoz]

previous velero deployment selector also match node-agent pods:

kubectl get pods --selector=app.kubernetes.io/instance=velero,app.kubernetes.io/name=velero
NAME                      READY   STATUS    RESTARTS       AGE
node-agent-4xp65          1/1     Running   0              22h
node-agent-gbsjf          1/1     Running   0              22h
…
velero-768f8b5b6d-2x79l   1/1     Running   0              22h

kubectl exec -it deploy/velero -c velero -- /velero version
Error from server (BadRequest): container velero is not valid for pod node-agent-hccs4

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Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped, please refer to the chart version instruction
• Title of the PR starts with chart name (e.g. [velero])