sweet: ensure go tool stderr is always preserved

This is the first step toward providing error output from subprocesses.

Change-Id: I8ffe0705397b4e9605979d0852a4eb031bb35651
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/383534
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
1 file changed