feat(cli): Allow building without self-update feature

[jirutka]

The self-update subcommand is undesirable if rustic is installed using the system package manager.

[simonsan]

I'm personally not a big fan of self-updating software, and I agree with the sentiment! I'm not sure though, we should completely build without it, the way I know it from e.g. rustup:

dev@TowerPC:/mnt/c/Users/dailyuse> rustup self update
error: self-update is disabled for this build of rustup
error: you should probably use your system package manager to update rustup

I like that more, so essentially being able to replace the command on build time with another implementation saying that it is being build without it and that there is a package manager involved. If we want, we could even make that configurable, so people can add their own message there via an environment variable or essentially just say that they could configure a package manager name (for people that have more than one on their system).

That needs to be documented in the https://github.com/rustic-rs/dev-docs as well, then.

[jirutka]

Okay, I changed it so the self-update subcommand is hidden and prints an error message when invoked.

[simonsan]

Thank you for the PR! 🚀 And a happy new year! 🥂

[alerque]

c.f. #1139