runtime: deprecate GOROOT

Fixes #51473.

Change-Id: Ic868790f480b21b472c98b39be797bcffb1fa892
Reviewed-on: https://go-review.googlesource.com/c/go/+/564142


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>