packer: add workflow to automatically update orka images

[gdams]

Adds support for auto regenerating the Orka macOS images on PR merge. For now while we don't have an accurate way of installing xcode this will only update the top layer

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