There are several ways to solve this in Go. The first that comes to mind, assuming you want to truncate to the first 12 runes, not bytes:

        func main() {
            v := []rune(os.Args[1])
            if len(v) > 12 {
                v = v[:12]
            }
            fmt.Println(string(v))
        }

Or more in the spirit of the C example in the post:

        func main() {
                res := make([]rune, 12)
                copy(res, []rune(os.Args[1]))
                fmt.Println(string(res))
        }

Note that res will stay on the stack, just like C.

I expect the author is trying to say something about Go that I'm not quite getting. Perhaps that it is not an expression-based language, so to make code readable you need to make use of multiple statements. That's by design, but I understand it may be unappealing if you want to program in an expression-heavy style.