ci: Don't annotate expected ssh_dispatch_run_fatal messages

[def-]

Seen in https://buildkite.com/materialize/nightly/builds/8192

zippy-ssh-bastion-host-1 | ssh_dispatch_run_fatal: Connection from 172.16.16.9 port 51480: Broken pipe [preauth]

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes: