rustbuild: Fix a typo with the Cargo book

The usage of `Path::new` prevented out-of-tree builds (like the bots do) from
working by accident!

Closes #46195

1 files changed, 1 insertions, 1 deletions

diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index 9186699a3b5..3c12cfc4c7f 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -159,7 +159,7 @@ impl Step for CargoBook {
 
         let target = self.target;
         let name = self.name;
-        let src = PathBuf::from("src/tools/cargo/src/doc/book");
+        let src = build.src.join("src/tools/cargo/src/doc/book");
 
         let out = build.doc_out(target);
         t!(fs::create_dir_all(&out));