Bootstrap command refactoring: quality-of-life improvements (step 4) Continuation of https://github.com/rust-lang/rust/pull/127120. This PR simply introduce two new functions (`BootstrapCommand:run` and `command`) that make it a bit easier to use commands in bootstrap. It also adds several `#[must_use]` annotations. This shouldn't (hopefully) have any effect on behavior. Especially the first commit IMO makes any code that runs commands more readable, and allows using the API in a fluent way, without needing to jump back and forth between the command and the `Build(er)`. Tracking issue: https://github.com/rust-lang/rust/issues/126819 r? `@onur-ozkan`