feat(cli/self-update): refine wording of "already installed Rust" warning

[rami3l]

Closes #3679 by mentioning https://rust-lang.github.io/rustup/installation/already-installed-rust.html in the rustup-init installer's output.

As discussed in #3679 (comment), if a rustup packager chooses to skip the installer completely, it is the responsibility of the packager to issue this warning in the package manager instead.

[mobiuscog]

Would this cause confusion if:

1. Rustup is installed elsewhere (system) but not in the user override
2. Rustup is installed in the user override already
   Or would the code checks prevent this text being shown in the case of (2) ?

I like the change though (as a non-reviewer).

[rami3l]

Would this cause confusion if:

1. Rustup is installed elsewhere (system) but not in the user override
2. Rustup is installed in the user override already
   Or would the code checks prevent this text being shown in the case of (2) ?

@mobiuscog My apologies but I am not sure if I have fully comprehended the said confusion.

Rustup is designed as a per-user application, but the way it's packaged in say Debian, as long as the package is installed by the sysadmin, rustc and cargo will be present in your PATH immediately; it's the toolchain installation that will happen locally in your user account.

Whether that is the case or you have previously installed rustup from https://rustup.rs doesn't matter; in both cases there's no need to run the installer again 👀

Maybe I have misunderstood something here?

[mobiuscog]

Apologies @rami3l - perhaps it's my confusion around what happens if rustup is already installed at a system level and I install a toolchain - I would have expected it to be at a system level as well (and therefore collide) but if the system rustup is per-user and would simply add to a local user toolchain install (and not affect the system install) then that's likely fine.

I have only ever installed it as a user, never as root. I have just noticed that rustup and rust are often different packages in a repository, and so there could be confusion around what happens if a user installs rustup via the shell, and then a later system update installs it as a system package. Perhaps I'm overthinking things here and just don't understand the difference between rustup as a distro package and rustup via the website/script, if at all there is one.

[rami3l]

I have only ever installed it as a user, never as root. I have just noticed that rustup and rust are often different packages in a repository, and so there could be confusion around what happens if a user installs rustup via the shell, and then a later system update installs it as a system package. Perhaps I'm overthinking things here and just don't understand the difference between rustup as a distro package and rustup via the website/script, if at all there is one.

@mobiuscog No worries, thanks again for your feedback and your prudence!