zsh: move aliases definitions after initExtra

Same motivation as in https://github.com/NixOS/nixpkgs/pull/28378.
zsh.initExtra parameter can be used by external modules which can
redefine user aliases. This change will give user-defined aliases
the highest priority.

modules/programs/zsh.nix

@@ -241,9 +241,9 @@ in
           source "$HOME/.zsh/plugins/${plugin.name}/${plugin.file}"
         '') cfg.plugins)}
 
-        ${aliasesStr}
-
         ${cfg.initExtra}
+
+        ${aliasesStr}
       '';
     })
     (mkIf cfg.oh-my-zsh.enable {