[installer] agave installer requires rustup but does not check for it

[nickfrosty]

Problem

The agave cli installer script ultimately requires Rustup to be installed on the user's system in order to link/name the platform tools as the solana toolchain on the user's system

If the user does not have rustup installed (like if they installed rust via brew install rust) the solana toolchain will not exist on their computer, and ultimately fail and result in errors like the following:

error: no such command: `+solana`

        Cargo does not handle `+toolchain` directives.
        Did you mean to invoke `cargo` through `rustup` instead?

• using rustup command for platform tools here
• see this issue here on the Anchor lang repo

Proposed Solution

• add a check for rustup in the agave installer script