sweet: run commands without Stderr set with Output

This way any *exec.ExitError that gets returned will have a populated
Stderr field.

Change-Id: I9aabe5f81bc1ff80b2bf4e304817087e36205b34
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/384514
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>