idea: CLI update option

Rather than needing to run curl -sL https://run.linkerd.io/install | sh to get a new version of the CLI, it would be nice if the CLI provided a mechanism for updating itself (e.g. linkerd cli-update).

A naive implementation would just be to wrap curl -sL https://run.linkerd.io/install | sh into something like linkerd cli-update. That might be a suitable first step. Some things to consider:

• Following semver, any minor version updates within the same major version should be backwards compatible and safe for the end user. Major version updates, if any, should potentially have a confirmation from the end user before proceeding. It's not clear if this is a use case that needs considered for the foreseeable future.
• By default, running curl -sL https://run.linkerd.io/install | sh installs the linkerd CLI to $HOME/.linkerd2/bin. However, the end user may have moved the CLI to a different location, so at minimum a cli-update command should verify $HOME/.linkerd2/bin exists before proceeding. If it doesn't exist, it's probably best not to "update" a new version there. Maybe the update logic should attempt to locate where linkerd is at (which linkerd?), or prompt the user to provide a location?
• Anything else?

Start with the proposed linkerd cli-update entry point and the existing curl -sL https://run.linkerd.io/install | sh installer flow. Clarify version-update behavior, confirmation for major releases, and how to handle $HOME/.linkerd2/bin or the path returned by which linkerd; done means the CLI can safely update itself without overwriting an unknown location.