dl: add go1.13.3 and go1.12.12

Go 1.13.3 and 1.12.12 have been cut, add commands for them.

Change-Id: I829fcedc392163c00739aef2e9ac285bd7ec085a
Reviewed-on: https://go-review.googlesource.com/c/dl/+/201641
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
2 files changed