improve bootstrap change-tracking system This PR aims to improve how change-tracking system works for bootstrap changes by doing the followings: - Enforce embedding directive informations about the changes on `bootstrap/src/lib.rs`. - Give more informative change inputs on the terminal (https://github.com/rust-lang/rust/pull/117815#discussion_r1390239657). - Avoid spamming the change informations(by reading and creating `.last-warned-change-id` under build output dir). see the zulip conversation for more details: https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/config.2Etoml.20change.20tracking cc `@RalfJung`