cmd/bent: minor cleanup in cronjob.sh

Change-Id: Ifec84d76ddacbe460237188328e3479a3304168c
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/373134
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: David Chase <drchase@google.com>