cmd/go: disallow go env -w GOROOT=...

The go command determines GOROOT from where the binary is.
It is an error to try to run a go command with a mismatched Go tree.
The only possible use for 'go env -w GOROOT=...' would be if you
copied the go executable to a new place (say, $HOME/bin) and needed
to set the default GOROOT so that it could find its way back home.
Don't do that. Use a symlink or a tiny shell script.

Fixes #77825.

Change-Id: I2b4430c46a9ef35fccb7406ffba3e44b3de191be
Reviewed-on: https://go-review.googlesource.com/c/go/+/749461
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/go/internal/envcmd/env.go

@@ -604,7 +604,17 @@ func getOrigEnv(key string) string {
 
 func checkEnvWrite(key, val string) error {
 	switch key {
-	case "GOEXE", "GOGCCFLAGS", "GOHOSTARCH", "GOHOSTOS", "GOMOD", "GOWORK", "GOTOOLDIR", "GOVERSION", "GOTELEMETRY", "GOTELEMETRYDIR":
+	case "GOEXE",
+		"GOGCCFLAGS",
+		"GOHOSTARCH",
+		"GOHOSTOS",
+		"GOMOD",
+		"GOROOT",
+		"GOTELEMETRY",
+		"GOTELEMETRYDIR",
+		"GOTOOLDIR",
+		"GOVERSION",
+		"GOWORK":
 		return fmt.Errorf("%s cannot be modified", key)
 	case "GOENV", "GODEBUG":
 		return fmt.Errorf("%s can only be set using the OS environment", key)

src/cmd/go/testdata/script/env_write.txt

@@ -71,6 +71,8 @@ stderr 'unknown go command variable GOGC'
 stderr 'GOEXE cannot be modified'
 ! go env -w GOVERSION=customversion
 stderr 'GOVERSION cannot be modified'
+! go env -w GOROOT=./go
+stderr 'GOROOT cannot be modified'
 ! go env -w GOENV=/env
 stderr 'GOENV can only be set using the OS environment'
 ! go env -w GODEBUG=gctrace=1