sweet: allow zero runs of benchmarks

This CL allows the user to only compile the benchmarks.

I found this useful when I am only collecting compilation statistics.
Context: CL 711260.

Change-Id: If615759f2ac2f3fa30839accdfb6fcbb2100ecfa
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/724920
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>