Read from initial stdin in Go?

I would like to read from the original stdin of a Go program. For example, if I did echo test stdin | go run test.go, I would want to have access to "test stdin". I've tried reading from os.Stdin, but if there's nothing in it, then it will wait for input. I also tried checking the size first, but the os.Stdin.Stat().Size() is 0 even when input is passed in.

What can I do?