scalability test: Retry before marking as regression

[def-]

See https://materializeinc.slack.com/archives/C01LKF361MZ/p1772530033342129
Scalability framework has been kind of flaky for a while, so it's probably time to do something. I would have preferred if we could make Materialize's performance more predictable instead!
Test run: https://buildkite.com/materialize/nightly/builds/15436

[github-actions]

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