cmd/bent: fix benchsize command

it did not work very well on Darwin as of Sonoma 14.6
and actually it no longer worked right on Linux, either.

Change-Id: Ib130223a64fa91c1141c0bf0cdb9da5171030540
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/609615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>