cmd/go: make use of Runnable method

Reported via unsupported Github pull request: #9299

Change-Id: I0e98dd68cbc68fcc6bcec15c5b33f20b6a861ec6
Reviewed-on: https://go-review.googlesource.com/3025Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/main.go

@@ -153,7 +153,7 @@ func main() {
 	}
 
 	for _, cmd := range commands {
-		if cmd.Name() == args[0] && cmd.Run != nil {
+		if cmd.Name() == args[0] && cmd.Runnable() {
 			cmd.Flag.Usage = func() { cmd.Usage() }
 			if cmd.CustomFlags {
 				args = args[1:]