sweet: limit build parallelism for cockroachdb

This is an attempt to get the PGO performance builders working.

Change-Id: Id4d2e5f59da323554a98aca45528efbcee9618a2
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/631678
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>