fix(ras-acc): apply wp-block-button class to checkout buttons

[chickenn00dle]

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:

Closes https://app.asana.com/0/1207817176293825/1208327956169310/f

This PR fixes an issue where checkout button blocks were losing the wp-block-button class when upgrading to epic/ras-acc.

How to test the changes in this Pull Request:

1. Make sure trunk is checked out and compiled
2. Add a checkout button to any post or page
3. As a reader view the post or page and verify the checkout button has the wp-block-button class as pictured above.
4. Checkout epic/ras-acc and build
5. Once again as a reader view the post or page with the checkout button. The wp-block-button class should be missing.
6. Repeat the above steps, but instead of epic/ras-acc checkout this branch on step 4. This time the wp-block-button class should not be missing.

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?