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

crates/eval/src/eval.rs

@@ -52,10 +52,10 @@ struct Args {
     #[arg(long, value_delimiter = ',', default_value = "rs,ts")]
     languages: Vec<String>,
     /// How many times to run each example.
-    #[arg(long, default_value = "1")]
+    #[arg(long, default_value = "8")]
     repetitions: usize,
     /// Maximum number of examples to run concurrently.
-    #[arg(long, default_value = "10")]
+    #[arg(long, default_value = "4")]
     concurrency: usize,
 }