steps/toolbx: Don't self-update and don't send notifications

[har7an]

This PR refines the toolbx update step, which is currently implemented as entering into the target toolbx container and then executing the running topgrade executable from there. The following changes to topgrades behavior in the toolbx container are made:

• Don't perform anther topgrade self-update (because the "outer" topgrade will have done that before)
• Don't show "update finished" notifications (because this is confusing and leads users to believe that topgrade finished rather quickly)

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

[SteveLauC]

Thanks!