`pixi shell` should restart when lockfile changes

[wolfv]

We should implement a watch-hook or something so that pixi shell restarts anytime the lockfile/environment changes.

The problem is that new packages might need to run activation scripts, and the PATH needs to be "re-hashed" for bash and zsh.

We currently have some fixes for bash and zsh, but a proper fix would also work for any other shell. Restarting is probably the most effective way of achieving this.