cmd/bench: make bent run with randomized builds

Sometimes a specific layout of the built binary may affect small
benchmark results, making false regression alarms.

This CL sets bent to be run with randomized builds to mitigate
this issue.

Change-Id: I225018bdc3fb824b726895833852af572b58190c
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/786660
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>