cmd/go: env changed flag respects $GOROOT/go.env
From $GOROOT/go.env file got GOPROXY and GOSUMDB and GOTOOLCHAIN default value.
Fixes #67542
Change-Id: I0cb2e1ab6a32963288ae463a9b0bd92ac6719447
GitHub-Last-Rev:
fda9be48b9e3bd3b124648eec1ef4ecfccac6d50
GitHub-Pull-Request: golang/go#67564
Reviewed-on: https://go-review.googlesource.com/c/go/+/587160
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>