Rollup merge of #120475 - Nilstrieb:cargo-build-my-a-, r=michaelwoerister

Improve error message when `cargo build` is used to build the compiler

Inspired by #76446.

Doing it for `core` is probably higher value but also way harder because tools like cargo or rustc-build-sysroot would need to be fixed first, which I don't feel like doing.

Author: GuillaumeGomez