Revise activation of mise shell with shell.

What does this merge request do and why?

Encountered an error when trying to activate mise with the /bin/zsh shell. The usage of mise activate $SHELL didn't work well with the /etc/shells.

nweinshenker@Nathans-MacBook-Pro-2 gdk % eval "$(mise activate $SHELL)"
error: invalid value '/bin/zsh' for '[SHELL_TYPE]'
  [possible values: bash, fish, nu, xonsh, zsh]

For more information, try '--help'.
nweinshenker@Nathans-MacBook-Pro-2 gdk % echo $SHELL
/bin/zsh
nweinshenker@Nathans-MacBook-Pro-2 gdk % eval "$(mise hook-env)"    
nweinshenker@Nathans-MacBook-Pro-2 gdk % mise cd ..
error: unrecognized subcommand 'cd'

Usage: mise [OPTIONS] <COMMAND>

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.