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

.github/workflows/eval.yml

@@ -69,7 +69,7 @@ jobs:
         run: cargo build --package=eval
 
       - name: Run eval
-        run: cargo run --package=eval -- --repetitions=3 --concurrency=1
+        run: cargo run --package=eval -- --repetitions=8 --concurrency=1
 
       # Even the Linux runner is not stateful, in theory there is no need to do this cleanup.
       # But, to avoid potential issues in the future if we choose to use a stateful Linux runner and forget to add code