Rollup merge of rust-lang#78059 - JohnTitor:fix-linkcheck, r=ehuss

Set `MDBOOK_OUTPUT__HTML__INPUT_404` on linkchecker

This is found in rust-lang/nomicon#240.
It seems the spurious failure shows up without this flag.

Author: JohnTitor

src/tools/linkchecker/linkcheck.sh

@@ -34,6 +34,9 @@ then
     exit 1
 fi
 
+# Avoid failure caused by newer mdbook.
+export MDBOOK_OUTPUT__HTML__INPUT_404=""
+
 book_name=""
 # Iterative will avoid cleaning up, so you can quickly run it repeatedly.
 iterative=0