Backport of CSI: fix handling of garbage collected node in node unpublish into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #12350 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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Fixes #12346 (ref #12346 (comment))

When a node is garbage collected, we assume that the volume is no
longer attached to it and ignore the ErrUnknownNode error. But we
used errors.Is to check for a wrapped error, and RPC flattens the
errors during serialization. This results in an error check that works
in automated testing but not in real clusters. Use a string contains
check instead.

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[lgfa29]

Manually backported

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