[Feature] rzup more aware of existing installs

[nuke-web3]

Feature

Presently re-running rzup will attempt to install everything again, and it may not need to. I would like the tool to better detect what already is installed and report what is discovered, perhaps with an option to brute force re-install / update with a CLI flag.

Motivation

• For an immediate re-run of the tool, I would want it to "fail" fast with status information rather than restart larger processes and downloads.

Implementation

• Still seems like a reasonable fit for a bash script with a bit more interactivity, as a bonus for UX use an interactive terminal menu for varients if no flag is set.
  • Inspiration for the bash bootstrap: https://github.com/rust-lang/rustup/blob/master/rustup-init.sh
• Consider making a very simple rust cli tool and install via cargo install rzup rather just extend cargo risczero if we need to boostrap via rust anyway.

Happy to put in a PR here if others think this is valuable. Related to #1813 #1815

[linear]

ZKVM-430 [Feature] `rzup` more aware of existing installs

[flaub]

It'd be really nice for it to handle multiple versions like other *up tools.

[nuke-web3]

Inspiration for the bash bootstrap: https://github.com/rust-lang/rustup/blob/master/rustup-init.sh , with a vision to have similar UI/UX for rzup as there is for rustup and also to be rust agnostic for a future where the RISC Zero toolchain is integrated more natively with more languages 🌅