Add packer build

QA

The following pipeline created the following image:

$ gcloud compute images list --filter runner    
 
NAME                                               PROJECT              FAMILY  DEPRECATED  STATUS
runners-windows-2019-core-containers-dev-df71503c  group-verify-df9383                      READY

After we run the cleanup pipeline we can see that the image was deleted

$ gcloud compute images list --filter runner   
Listed 0 items.

closes gitlab-org/gitlab-runner#4621 (closed)