Incorporate self-updating with new `hc update` command.

[alilleybrinker]

cargo-dist automatically generates a self-updater, and it would be nice if we could leverage it, if present, to allow an easy path for users to self-update Hipcheck.

The basic idea would be to add an hc update command which uses the auto-generated and installed hc-update CLI produced and distributed automatically with the installer cargo-dist produces.

See here for more: https://opensource.axo.dev/cargo-dist/book/installers/updater.html