What does this PR change? What problem does it solve?

dump flag --target should be allowed to write existing file

The primary objective of #4678 is to restore a block PVC.
steps:

1. mount the block pvc to a pod as volumeDevice
2. start the pod, write data to the block device inside the pod using restic

The block device is already there, we need to use restic dump to write data into it

Was the change previously discussed in an issue or on the forum?

Closes #4678

Checklist

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