Bootstrap outputs some progress messages on stdout

[clubby789]

I tried this:

x b -h 2>/dev/null

I expected to see this happen: No status messages, only help output

Instead, this happened:

info: Downloading and building bootstrap before processing --help command.
      See src/bootstrap/README.md for help with common commands.
extracting /home/jamie/Documents/rust/build/cache/2023-04-20/rust-std-beta-x86_64-unknown-linux-gnu.tar.xz
extracting /home/jamie/Documents/rust/build/cache/2023-04-20/rustc-beta-x86_64-unknown-linux-gnu.tar.xz
extracting /home/jamie/Documents/rust/build/cache/2023-04-20/cargo-beta-x86_64-unknown-linux-gnu.tar.xz
Building bootstrap

This makes stdout awkward to parse when using e.g. --json-output.

(I'll fix this myself later today if no one else wants to claim it)
@rustbot claim
@rustbot label +T-bootstrap