[Bug] Fix unset_contextvars to unset the right contextvars

[gshank]

Description

This comes from #dbt-core/8866, an error in the logs in which the Hook node_info persists into the next dbtRunner call. This was happening for two reasons: 1) we were not using the log_contextvars context manager in task/run.py so the node_info was not getting reset at the end of run_hook and 2) the unset_contextvars functions was not working correctly. The fix in this PR is to fix unset_contextvars.

Solution

Properly construct the prefix_key to unset the contextvars.