fix: use pkgs.k8s.io apt mirror (previous fails)

[displague]

part of #62

With this change the K8s CPs successfully install packages during cloud-init:

/var/log/cloud-init-output.log:

...
[addons] Applied essential addon: CoreDNS
[addons] Applied essential addon: kube-proxy

Your Kubernetes control-plane has initialized successfully!

To start using your cluster, you need to run the following as a regular user:
...

root@k8s-cluster1-pool1-cp-1:/var/log# kubectl get nodes -o wide 
NAME                      STATUS     ROLES           AGE     VERSION   INTERNAL-IP     EXTERNAL-IP   OS-IMAGE             KERNEL-VERSION      CONTAINER-RUNTIME
k8s-cluster1-pool1-cp-1   NotReady   control-plane   5m56s   v1.27.5   147.28.197.41   <none>        Ubuntu 20.04.6 LTS   5.15.0-97-generic   containerd://1.6.28

[displague]

The most recent CI timed-out - in that commit the worker was updated to use the new apt repos.
Without that change, the previous commit passed, a sign that #55 isn't too far away.

[dcallao]

@cprivitere Would take a look at this when you have bandwidth? This LGTM.