nixos/installer: ship only one mbrola voice per language

This reduces the size of mbrola-voices by 387M (647M -> 260M) for all
installers that ship with speechd, to make sure that they fit the output
limits of hydra while not compromising too much on accessbility.

nixos/modules/installer/cd-dvd/installation-cd-graphical-base.nix

@@ -66,4 +66,13 @@ with lib;
     glxinfo
   ];
 
+  nixpkgs.overlays = [
+    (self: super: {
+      mbrola-voices = super.mbrola-voices.override {
+        # only ship with one voice per language
+        languages = [ "*1" ];
+      };
+    })
+  ];
+
 }