Replace bt self update with bt update command

[Parker Henderson (parkerhendo)]

Improve the discoverabilty of the native updater by making it a top-level command.

[Ankur Goyal (ankrgyl)]

should we keep bt self update as an alias?

[github-actions]

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[Parker Henderson (parkerhendo)]

should we keep bt self update as an alias?

​ah, you're right. Reimplemented to keep (but hide) the self update command in favor of bt update

[Cedric / ViaDézo1er (viadezo1er)]

Maybe update src/self_update.rs:142: "self-update is only supported for installer-based installs.\ncurrent executable: {}\nif this was installed with Homebrew/apt/choco/etc, update with that package manager", and README.md:398: - Add richer channel controls for self-update (for example pinned/branch canary selection).

I think you wanted to change the self-update wording, and there's some past slop to clean (we don't support brew/apt/choco, only curl ... | sh and npm).