Set GOROOT.

Before:

```
$ cat > example.go
package main
import "fmt"
func main() {
  fmt.Printf("Hi!\n")
}
$ go run example.go
example.go:2:8: cannot find package "fmt" in any of:
        /usr/local/go/src/pkg/fmt (from $GOROOT)
        ($GOPATH not set)
package runtime: cannot find package "runtime" in any of:
        /usr/local/go/src/pkg/runtime (from $GOROOT)
        ($GOPATH not set)
$ GOROOT=$GOENV_ROOT/versions/$(goenv version) go run example.go
Hi!
```

After:

```
$ cat > example.go
package main
import "fmt"
func main() {
  fmt.Printf("Hi!\n")
}
$ go run example.go
Hi!
```