fix: correct font family

[laurelfulford]

All Submissions:

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

Changes proposed in this Pull Request:

This PR fixes a tiny error I noticed while testing https://github.com/Automattic/newspack-blocks/pull/1847, but that wasn't introduced in it: the WooCommerce errors are not picking up the right font family.

How to test the changes in this Pull Request:

1. Navigate to Customizer > Typography, and set the Heading font to something noticeably different than the system UI font used by Newspack UI (I used Chalkboard).
2. Start a modal checkout using the Donate Block or Checkout Button block.
3. For the credit card number, use a number that will fail like 4000000000000002.
4. Note the font family used by the error:

5. Apply this PR and run npm run build.
6. Repeat steps 2-4 and confirm the error uses the right sans-serif font:

Other information:

• [x] 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 @adekbadek!