`x --help` builds bootstrap

[jyn514]

Summary

I want to know what arguments I can use with bootstrap. In a fresh checkout, that takes a while.

We can avoid this as follows:

1. Save the output of x --help to a file.
2. In the python wrapper, print that file if it exists.
3. In CI, bypass the python wrapper and check that the file is up-to-date.

Command used

./x --help

Expected behaviour

I see help output almost immediately.

Actual behaviour

INFO: Downloading and building bootstrap before processing --help command.
      See src/bootstrap/README.md for help with common commands.
downloading https://static.rust-lang.org/dist/2025-05-26/rust-std-beta-x86_64-unknown-linux-gnu.tar.xz
...

Bootstrap configuration (bootstrap.toml)

(empty)

Operating system

WSL running Ubuntu 24.04 on Windows 11

HEAD

52882f6

Additional context

Build Log

<log>

@rustbot label I-slow