fix(modal-checkout): allow metorik scripts

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

The Metorik (WC tracking) scripts have to be allowed to load on WC pages for the tracking to work.

How to test the changes in this Pull Request:

1. On the base branch, load the /checkout/?modal_checkout=1 page, observe the metorik JS is not loaded
2. Switch to this branch, observe it's loaded

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[laurelfulford]

Thanks for the catch and the fix, @adekbadek! 🙌