Book export doesn't print included content #2228

Describe the bug
When I export a page or a chapter, content that I include with {{@123}} tags is rendered in the output. However, the literal tag is printed in the output when I export the book.

Steps To Reproduce
Steps to reproduce the behavior:

1. Create a page that includes content from another page ({{@123}}).
2. Export the book containing the page from step 1.
3. The exported file prints the literal include tag.

Expected behavior
The page and chapter exports both print the rendered, included content instead of the literal include tag. I expected the book export to do the same.

Screenshots
Rendered page with an include tag:

Page source showing include tag:

Exported page:

Exported book:

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): 0.29.3 (screenshots are from the demo site)
• PHP Version: unknown
• Hosting Method (Nginx/Apache/Docker): unknown