goinstall: honour -install=false flag when -make=true

R=rsc
CC=golang-dev
https://golang.org/cl/5448124

src/cmd/goinstall/make.go

@@ -29,7 +29,12 @@ func domake(dir, pkg string, tree *build.Tree, isCmd bool) (err error) {
 	} else if *clean {
 		cmd = append(cmd, "clean")
 	}
+	if *doInstall {
 		cmd = append(cmd, "install")
+	}
+	if len(cmd) <= 3 { // nothing to do
+		return nil
+	}
 	return run(dir, makefile, cmd...)
 }