sweet: discard stderr from List and enforce GOROOT in common.Go

This change resolves two issues in how Sweet runs on the builders:

1. List calls CombinedOutput to get its output, but the Go tool may
   write "go: downloading ..." lines to stderr, breaking callers.
   To fix this, call Output instead.

2. Like how cmd/bench needs to enforce the right GOROOT because the one
   from the environment may be wrong for some toolchain, Sweet neglected
   to do this too. So, set the right GOROOT for any common.Go that's
   created in Sweet.

Together, these fixes should allow Sweet to run on the perf builder
without issue.

Change-Id: Iaa4a87ba6579681149e6faa51cac1798e16e4d8a
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/383294
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>
3 files changed