canonical-k8s: download also the snapd snap for air gapped or proxy scenarios

This change simplifies the maintainance in the future for the ck8s image, as one should only change the snap version number, the only that pins the Kubernetes version.

Also, snapd snap is now downloaded as well, to not require the download in airgapped/http proxied environments. Otherwise, the snap daemon needs to also have a proxified systemd setting, and we would like to avoid that.