rustenv should probably silence some of the rustup output

[asottile]

currently it produces output like this:

$ rustenv renv
info: downloading installer
info: profile set to 'default'
info: default host triple is x86_64-unknown-linux-gnu
info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'
info: latest update on 2022-08-11, rust version 1.63.0 (4b91a6ea7 2022-08-08)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-docs'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-docs'
 18.3 MiB /  18.3 MiB (100 %)   9.5 MiB/s in  1s ETA:  0s
info: installing component 'rust-std'
 26.1 MiB /  26.1 MiB (100 %)  14.4 MiB/s in  1s ETA:  0s
info: installing component 'rustc'
 54.3 MiB /  54.3 MiB (100 %)  15.9 MiB/s in  3s ETA:  0s
info: installing component 'rustfmt'
info: default toolchain set to 'stable-x86_64-unknown-linux-gnu'

  stable-x86_64-unknown-linux-gnu installed - rustc 1.63.0 (4b91a6ea7 2022-08-08)


Rust is installed now. Great!

To get started you need Cargo's bin directory (/tmp/explains/renv/rust/bin) in 
your PATH
environment variable. This has not been done automatically.

To configure your current shell, run:
source "/tmp/explains/renv/rust/env"

the last little bit there is especially problematic since it's not how rustenv is intended to work