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>

api/next/51473.txt

+pkg runtime, func GOROOT //deprecated #51473

doc/next/6-stdlib/99-minor/runtime/51473.md

+The [GOROOT] function is now deprecated.
+In new code prefer to use the system path to locate the “go” binary,
+and use `go env GOROOT` to find its GOROOT.

src/runtime/extern.go

@@ -336,6 +336,11 @@ var defaultGOROOT string // set by cmd/link
 // GOROOT returns the root of the Go tree. It uses the
 // GOROOT environment variable, if set at process start,
 // or else the root used during the Go build.
+//
+// Deprecated: The root used during the Go build will not be
+// meaningful if the binary is copied to another machine.
+// Use the system path to locate the “go” binary, and use
+// “go env GOROOT” to find its GOROOT.
 func GOROOT() string {
 	s := gogetenv("GOROOT")
 	if s != "" {