solana-install init doesn't know when there's no work to do

[garious]

Problem

solana-install init doesn't know when there's no work to do.

Does a bunch of work:

solana-install init beta

Does the same work again:

solana-install init beta

Proposed Solution

Detect when there's no work to do and tell the user that they are already up-to-date.

[mvines]

This is the case when a named version is used, eg. solana-install init 0.22.2. For a release channel (edge/beta/stable), this issue is the case though. It doesn't attempt to first check if the latest build for the given release channel is what's currently installed.