[infra] Balance the number shards on the AOT builder with co19_2 tests
[infra] Balance the number shards on the AOT builder with co19_2 tests

The regular test shards run in about half the time. Therefore, we can
use less shards for overall efficiency without sacrificing cycle time.

TBR=asiva@google.com
Change-Id: Ib8b58a03f9c40d0e8f288cde6e148a54fc1ed1e5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138300
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk