python: Fix another removed f-string

[def-]

Forgot to switch the escaped {{}} strings which are sent to testdrive, follow-up to #18821

Fell over in https://buildkite.com/materialize/nightlies/builds/2242#0187a103-823b-4734-81ed-c91e454c3184

Motivation

• This PR fixes a previously unreported bug.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration, 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: