ci: add a check for non-vendored packages
ci: add a check for non-vendored packages

This check fails the lint-go CI job if nomad references a package that
is not vendored.

dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad