cmd/go: print the proper error message for setting telemetry vars

For #68960

Change-Id: I5495b3d45ad3817d7edf6336a5e24a331d5cf8d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/609115
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Sam Thanawalla <[email protected]>

Author: matloob

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

@@ -590,7 +590,7 @@ func getOrigEnv(key string) string {
 
 func checkEnvWrite(key, val string) error {
 	switch key {
-	case "GOEXE", "GOGCCFLAGS", "GOHOSTARCH", "GOHOSTOS", "GOMOD", "GOWORK", "GOTOOLDIR", "GOVERSION":
+	case "GOEXE", "GOGCCFLAGS", "GOHOSTARCH", "GOHOSTOS", "GOMOD", "GOWORK", "GOTOOLDIR", "GOVERSION", "GOTELEMETRY", "GOTELEMETRYDIR":
 		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/telemetry.txt

@@ -46,6 +46,6 @@ go env
 stdout 'GOTELEMETRY=''?on''?'
 stdout 'GOTELEMETRYDIR=''?'$userconfig'[\\/]go[\\/]telemetry''?'
 ! go env -w GOTELEMETRY=off
-stderr '^go: unknown go command variable GOTELEMETRY$'
+stderr '^go: GOTELEMETRY cannot be modified$'
 ! go env -w GOTELEMETRYDIR=foo
-stderr '^go: unknown go command variable GOTELEMETRYDIR$'
+stderr '^go: GOTELEMETRYDIR cannot be modified$'