Add nightly test of Regal against user repos

[charlieegan3]

We have a number of repos using Regal to lint policy. It'd be nice to be able to run a main build of Regal against these repos nightly.

Current list:

• Kubescape Rego Library
• Elastic Cloudbeat
• ScubaGear
• Red Hat COP Rego Policies
• GKE Policy Automation
• OPA Library
• Conftest
• Legitify
• Kics
• CVAT
• Brainiac
• MoJ Modernisation Platform
• Spacelift Policies Example Library
• Konstraint

Perhaps we could have some kind of yaml to describe the files to run it against:

repos:
- url: https://github.com/kubescape/regolibrary
  config: path
  run_path: path

or similar.