zsh: add completions to fpath

modules/programs/zsh.nix

@@ -107,6 +107,11 @@ in
         ${if cfg.history.ignoreDups then "setopt" else "unsetopt"} HIST_IGNORE_DUPS
         ${if cfg.history.share then "setopt" else "unsetopt"} SHARE_HISTORY
 
+        # Tell zsh how to find installed completions
+        for p in ''${(z)NIX_PROFILES}; do
+          fpath+=($p/share/zsh/site-functions $p/share/zsh/$ZSH_VERSION/functions)
+        done
+
         ${if cfg.enableCompletion then "autoload -U compinit && compinit" else ""}
         ${optionalString (cfg.enableAutosuggestions)
           "source ${pkgs.zsh-autosuggestions}/share/zsh-autosuggestions/zsh-autosuggestions.zsh"