`rustup toolchain install` doesn't consistently trigger self update

[rami3l]

When going through update()'s logic, it seems that both rustup update and rustup toolchain install foo will do one of the following:

1. Perform self-update based on runtime settings.
2. Issue a warning when self-update is disabled at compile time.

However this is not true for rustup toolchain install with no arguments (corresponding to the branch at L1021), since the first case is impossible:

rustup/src/cli/rustup_mode.rs

Lines 1017 to 1028 in 59aec45

	if self_update {
	exit_code &= self_update::self_update(&dl_cfg).await?;
	}
	} else if ensure_active_toolchain {
	let (toolchain, source) = cfg.ensure_active_toolchain(force_non_host, true).await?;
	info!("the active toolchain `{toolchain}` has been installed");
	info!("it's active because: {}", source.to_reason());
	} else {
	exit_code &= common::update_all_channels(cfg, opts.force).await?;
	if self_update {
	exit_code &= self_update::self_update(&dl_cfg).await?;
	}

This could be considered as an inconsistency.