cmd/bent: change to the single, latest version of goawk

Having two versions of the same benchmarks creates "noise"
because they benchmark the same packages.  Oops.

Change-Id: I2ede6e5543ab44f78be4e36e44d2b7877696887c
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/445637
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>