buildlet: change image name for COS-with-vmx buildlet
buildlet: change image name for COS-with-vmx buildlet

The COS image I'd forked from earlier didn't have CONFIG_KVM or
CONFIG_KVM_INTEL enabled in its kernel, so even though I'd enabled the
VMX license bit for the VM, the kernel was unable to use it.

Now I've instead rebuilt the ChromiumOS "lakitu" board with a modified
kernel config:

   https://cloud.google.com/container-optimized-os/docs/how-to/building-from-open-source

More docs later. Still tinkering. Nothing uses this yet.

Updates golang/go#15581 (solaris)
Updates golang/go#23060 (dragonfly)
Updates golang/go#30262 (riscv)
Updates golang/go#30267 (fuchsia)
Updates golang/go#23824 (android)

Change-Id: Id2839066e67d9ddda939d96c5f4287af3267a769
Reviewed-on: https://go-review.googlesource.com/c/163057
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build