dl: add go1.12.9

Go 1.12.9 has been cut¹, add a command for it.

¹ https://go.googlesource.com/go/+/refs/tags/go1.12.9

Change-Id: I7565d03fc859eb79afc9e477b947c972d9a3e09b
Reviewed-on: https://go-review.googlesource.com/c/dl/+/190411
Reviewed-by: Alexander Rakoczy <alex@golang.org>
1 file changed