ci: Improve fallback logic for benchmarks (again)

[def-]

Sorry, noticed another problem in https://buildkite.com/materialize/nightly/builds/15534#019cd8bd-bfc3-433d-bc12-6c831c3588ef

Test run: https://buildkite.com/materialize/nightly/builds/15539

[github-actions]

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[bosconi]

Looks good.