aerc: fix config paths on darwin

When xdg.enable is set, aerc uses the $XDG_CONFIG_HOME variable instead
of the default $HOME/Library/Preferences. home-manager needs to check
this to make sure aerc can find the proper file in both cases.

modules/programs/aerc.nix

@@ -24,7 +24,7 @@ let
   aerc-accounts =
     attrsets.filterAttrs (_: v: v.aerc.enable) config.accounts.email.accounts;
 
-  configDir = if pkgs.stdenv.isDarwin then
+  configDir = if (pkgs.stdenv.isDarwin && !config.xdg.enable) then
     "Library/Preferences/aerc"
   else
     "${config.xdg.configHome}/aerc";

tests/modules/programs/aerc/settings.nix

@@ -5,7 +5,7 @@ with lib;
 {
   config = {
     nmt.script = let
-      dir = if pkgs.stdenv.isDarwin then
+      dir = if (pkgs.stdenv.isDarwin && !config.xdg.enable) then
         "home-files/Library/Preferences/aerc"
       else
         "home-files/.config/aerc";