runtime/race: race detector misses when copying into an interface

The following is missed by the race detector 

```
var (
    a = "hi"
    c = make(chan struct{})
)

func f() {
    a = "bye"
    close(c)
}

func main() {
    go f()
    fmt.Println(a)
    // fmt.Printf(a)
    <-c
}
```

But changing the fmt call to `fmt.Printf` where the `a` value is copied into the format string triggers a race
