Rollup merge of #76800 - jyn514:usage, r=Mark-Simulacrum

Rollup merge of #76800 - jyn514:usage, r=Mark-Simulacrum

Don't generate bootstrap usage unless it's needed

Previously, `x.py` would unconditionally run `x.py build` to get the
help message. After https://github.com/rust-lang/rust/issues/76165,
when checking the CI stage was moved into `Config`, that would cause an
assertion failure (but only only in CI!):

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', src/bootstrap/config.rs:619:49
```

This changes bootstrap to only generate a help message when it needs
to (when someone passes `--help`).

r? @Mark-Simulacrum
This should fix the CI failures in https://github.com/rust-lang/rust/pull/76797 and https://github.com/rust-lang/rust/pull/75991.