packer: auto rebuild base image for intel

[gdams]

We can easily (and quickly) rebuild the base image for intel which means that it will always contain security updates etc. I've enabled this for intel but cannot make the same change for arm64 yet as it the Xcode download is too flaky.

Checklist

• [ ] commit message has one of the standard prefixes
• [ ] faq.md updated if appropriate
• [ ] other documentation is changed or added (if applicable)
• [ ] playbook changes run through VPC or QPC (if you have access)
• [ ] VPC/QPC not applicable for this PR
• [ ] for inventory.yml changes, bastillion/nagios/jenkins updated accordingly