Set `MDBOOK_OUTPUT__HTML__INPUT_404` on linkchecker
Set `MDBOOK_OUTPUT__HTML__INPUT_404` on linkchecker

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust