CI: Skip UCCL nixlbench tests (#2000)

## What?
Skip UCCL nixlbench test due to frequent hangs, see
https://github.com/ai-dynamo/nixl/issues/1999


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## Summary by CodeRabbit

* **Tests**
* Disabled flaky GPU-based UCCL transfer tests in CI pending an upstream
fix.
  * Other UCX tests and supporting test infrastructure remain unchanged.

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Signed-off-by: Ovidiu Mara <ovidium@nvidia.com>