[installer] require rustup on the system

[nickfrosty]

Problem

the agave installer ultimately requires having rustup installed in order to link the platform tools to become the solana toolchain, but does not check for it. so if a developer installs rust via brew or some other means, they may not have rustup available on their machine. and ultimately run into issues with the solana rust toolchain not being available

Summary of Changes

• add a check in the agave-install-init script for rustup
• reordered the install docs to have the prereqs closer to the top
• added rustup to the brew install command for prereqs

Fixes #4651

[steviez]

I think there is some nuance here for different groups of people who might be using the CLI:

• Developers
• "Regular" users who may just be doing simple stuff like using the CLI for transfers or native staking

The first group will absolutely want these tools, but the latter might be led to believe they need to install a bunch of things they don't need.