feat(rustup-mode): add no_update flag to rustup toolchain install

[nozwock]

Addresses #4505, adding a flag to don't try to update already installed toolchains.

[djc]

I think we'd want some tests for this.

[nozwock]

I think we'd want some tests for this.

What kinds? I can't think of too many.

#[tokio::test]
async fn toolchain_install_no_change_with_no_update() {
    let mut cx = CliTestContext::new(Scenario::None).await;

    {
        let cx = cx.with_dist_dir(Scenario::ArchivesV2_2015_01_01);
        cx.config
            .expect(["rustup", "toolchain", "add", "stable"])
            .await
            .is_ok();
    }

    let cx = cx.with_dist_dir(Scenario::SimpleV2);
    cx.config
        .expect(["rustup", "install", "--no-update", "stable"])
        .await
        .with_stdout(snapbox::str![[r#"

  stable-[HOST_TRIPLE] unchanged - 1.0.0 (hash-stable-1.0.0)


"#]])
        .with_stderr(snapbox::str![[""]])
        .is_ok();
}

Actually, I'd expected it to not be unchanged here without --no-update here, but it's still the same...

[djc]

Need to update the cli_ui tests. The test failures should give some hints on how to do this efficiently.