dl: add go1.12.7 and go1.11.12

Go 1.12.7 and 1.11.12 have been cut, add commands for them.

Change-Id: I2ef35f474ac0f88b8262d0cce15e9ff268436d72
Reviewed-on: https://go-review.googlesource.com/c/dl/+/185148
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
2 files changed