release-23.2: roachtest: use first transient error when checking for flakes

[renatolabs]

Backport 1/1 commits from #124403.

/cc @cockroachdb/release

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Previously, roachtest would only look at the outermost error in a chain that matched a TransientError (or ErrorWithOwnership) when checking for flakes. However, that is in most cases not what we want: if a transient error wraps another transient error, the actual reason for the failure is the original (wrapped) error.

Informs: #123887

Release note: None

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Release justification: test only change.