sweet: save build logs to .build.logs in result dir

This CL saves stdout and stderr during build to *.build.logs in the
result dir.

Change-Id: Ieb8c310a7cc62bb7210e2b8f5c9c29d561b8d401
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/709815
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>