Update Jenkins template example for supervisor builder

[dilyar85]

This PR adds the required steps in the supervisor builder Jenkins template file to get the apt-get update command working.

See https://kubernetes.io/blog/2023/08/15/pkgs-k8s-io-introduction/ for more details.

Verified internally using the updated Jenkins template:

==> vsphere-supervisor.vm: Creating temporary RSA SSH key for instance...
    vsphere-supervisor.vm: Connecting to Supervisor cluster...
    vsphere-supervisor.vm: Successfully connected to Supervisor cluster
    ...
==> vsphere-supervisor.vm: Provisioning with shell script: /var/folders/nf/cndlm4ts2bd9jz3dd667jfb80000gr/T/packer-shell350374452
    vsphere-supervisor.vm: deb [signed-by=/usr/share/keyrings/jenkins-keyring.gpg] https://pkg.jenkins.io/debian binary/
    vsphere-supervisor.vm: deb [signed-by=/usr/share/keyrings/kubernetes-apt-keyring.gpg] https://pkgs.k8s.io/core:/stable:/v1.28/deb/ /
    vsphere-supervisor.vm: Preconfiguring packages ...
    ...
    vsphere-supervisor.vm: done.
    ...
    vsphere-supervisor.vm: [sample-job] $ /bin/sh -xe /tmp/jenkins7660215003182937504.sh
    vsphere-supervisor.vm: + echo Hello VM-Service from Jenkins
    vsphere-supervisor.vm: Hello VM-Service from Jenkins
    vsphere-supervisor.vm: Finished: SUCCESS
    vsphere-supervisor.vm: Completed sample-job #1 : SUCCESS
    vsphere-supervisor.vm: Skip cleaning up the source objects as specified in config
    vsphere-supervisor.vm: Build 'vsphere-supervisor' finished.
Build 'vsphere-supervisor.vm' finished after 9 minutes 31 seconds.

[tenthirtyam]

lgtm

[nywilken]

LGTM