Auto merge of #127321 - Kobzol:bootstrap-cmd-refactor-4, r=onur-ozkan

Bootstrap command refactoring: quality-of-life improvements (step 4)

Continuation of rust-lang/rust#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: rust-lang/rust#126819

r? `@onur-ozkan`