rio: use XDG config for both linux and darwin

`rio` now uses the same config location for both Linux and macOS:

> MacOS and Linux configuration file path is `~/.config/rio/config.toml`.

Ref: https://raphamorim.io/rio/docs/configuration-file

tests/modules/programs/rio/example-settings.nix

@@ -1,13 +1,6 @@
 { config, pkgs, ... }:
 
 let
-  inherit (pkgs.stdenv.hostPlatform) isDarwin;
-
-  path = if isDarwin then
-    "Library/Application Support/rio/config.toml"
-  else
-    ".config/rio/config.toml";
-
   expected = pkgs.writeText "rio-expected.toml" ''
     cursor = "_"
     padding-x = 0
@@ -26,7 +19,7 @@ in {
   };
 
   nmt.script = ''
-    assertFileExists home-files/"${path}"
-    assertFileContent home-files/"${path}" '${expected}'
+    assertFileExists home-files/.config/rio/config.toml
+    assertFileContent home-files/.config/rio/config.toml '${expected}'
   '';
 }