rustbuild: Fix a typo with the Cargo book
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

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