build: move Linux and nacl builders to Kubernetes, add ssacheck trybot

And shard the nacl builders.

Fixes golang/go#15242
Fixes golang/go#12546

Change-Id: I3311678dbb64a76026f75f5691cffac0f25d976b
Reviewed-on: https://go-review.googlesource.com/22780
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ross Light <light@google.com>
2 files changed