nixos/zsh-syntax-highlighting: Fix highlighting when ohMyZsh is enabled

From https://github.com/zsh-users/zsh-syntax-highlighting#faq:

"It must be sourced after all custom widgets have been created
(i.e., after all zle -N calls and after running compinit)."

zsh-syntax-highlighting must be sourced to the end.

nixos/modules/programs/zsh/zsh-syntax-highlighting.nix

@@ -81,7 +81,7 @@ in
     ];
 
     programs.zsh.interactiveShellInit = with pkgs;
-      lib.concatStringsSep "\n" ([
+      lib.mkAfter (lib.concatStringsSep "\n" ([
         "source ${zsh-syntax-highlighting}/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh"
       ] ++ optional (length(cfg.highlighters) > 0)
         "ZSH_HIGHLIGHT_HIGHLIGHTERS=(${concatStringsSep " " cfg.highlighters})"
@@ -95,6 +95,6 @@ in
             styles: design:
             "ZSH_HIGHLIGHT_STYLES[${styles}]='${design}'"
           ) cfg.styles)
-      );
+      ));
   };
 }