maybe publish bottlerocket-variant

## Description

Consider publishing this as a separate tool and/or lib.

> sources/bottlerocket-variant
>
> I expect this will go away as we eliminate variant-specific builds of anything. If that matches your understanding, I'm cool with this being duplicated as well. Otherwise, potentially we could turn it into a CLI tool that build scripts can invoke.

_Originally posted by @bcressey in https://github.com/bottlerocket-os/twoliter/issues/37#issuecomment-1692429512_

## Definition of Done

When `bottlerocket-variant` has been published to crates.io and Bottlerocket and Twoliter are both using it, **OR**, when we have decided not to do so.

Start with the `sources/bottlerocket-variant` component and determine whether it should become a separately published Rust library, a CLI invoked by build scripts, or remain duplicated. Review how Bottlerocket and Twoliter currently use it, then seek a project decision. Done means either it is published to crates.io and both projects use it, or the project records a decision not to publish it.

Written by the indexing model from the issue text.