Add policy set definition workaround

[jaredfholgate]

Overview/Summary

This fixes an issue where policy set definitions only reference one policy definition.

I tried various workarounds in the class definitions before resorting this option. It appears there is a fundamental issue in PowerShell that results in needing to fix it like this rather than in the class constructor / definition as you would expect.

This PR fixes/adds/changes/removes

N/A

Breaking Changes

N/A

Testing Evidence

This is a fix to a script library used for policy sync, so does not need portal testing.

You can see a working run based on this branch over here: https://github.com/Azure/terraform-azurerm-caf-enterprise-scale/pull/972/files

As part of this Pull Request I have

• Checked for duplicate Pull Requests
• Associated it with relevant issues, for tracking and closure.
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Ensured contribution guidance is followed.
• Updated relevant and associated documentation.
• Updated the "What's New?" wiki page (located: /docs/wiki/whats-new.md)