dl: add go1.12.8 and go1.11.13

Go 1.12.8 and Go 1.11.13 have been released┬╣, add commands for them.

┬╣ https://groups.google.com/d/msg/golang-announce/65QixT3tcmg/DrFiG6vvCwAJ

Change-Id: Ic70cc930f2a34aff096274a036470c37c930cd68
Reviewed-on: https://go-review.googlesource.com/c/dl/+/190178
Reviewed-by: Filippo Valsorda <filippo@golang.org>
2 files changed