Previously, we were writing binaries to GOROOT without ever reading them.

Updates #28387

Change-Id: Ic84999f46a367fcbf2ae1eac8bfdcbceb24dc748
Reviewed-on: https://go-review.googlesource.com/c/go/+/207342
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>