Switch to go v1.12.8

[ibuildthecloud]

Due to golang/go#33405 containerd will leak fds as reported here containerd/containerd#3481.

As soon as go 1.12.8 is released we should update to it to fix this issue.

[warmchang]

go1.12.8 is out https://groups.google.com/forum/#!topic/golang-announce/65QixT3tcmg

[ibuildthecloud]

1.12.8 was a security release, we need 1.12.9 which is out today.