Use safer append to hook function arrays (#8406)

Use add-zsh-hook to add functions to hooks. That way they won't be added again
when doing `source ~/.zshrc` multiple times.

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

plugins/pipenv/pipenv.plugin.zsh

@@ -23,7 +23,8 @@ _togglePipenvShell() {
     fi
   fi
 }
-chpwd_functions+=(_togglePipenvShell)
+autoload -U add-zsh-hook
+add-zsh-hook chpwd _togglePipenvShell
 
 # Aliases
 alias pch="pipenv check"