Make the default repetitions be 8, and concurrency 4 (#29576)

This is based on having observed that there is a lot of variation
between runs on `n=1` and `n=3`.

* With `n=8` two runs on the same branch give answers that seem close
enough to be reasonably consistent.
* With higher concurrency, trying to run this many repetitions seems to
lead language servers to time out a lot, causing evals to fail.

Release Notes:

- N/A
04c68dc0cfRichard Feldman committed on 4/30/2025, 7:21:19 PM· committed by GitHubparent399eced
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