Fix: Custom fonts in FSE using admin domain

[cameronterry]

This PR includes:

• Fixes an issue where custom fonts would use the admin domain in Full Site Editing/Blocks, instead of the primary domain.
• This, can depending on server configuration, cause Cross-Origin Request Block errors and the fonts not to be used correctly.
• Mainly impacts those mapping use of the WP-Admin > Appearance > Editor > Styles > Typography > Manage Fonts and either uploading or using the Install Fonts/Google Fonts options.