nixos: flakes.nix -> nix-flakes.nix

I guess this is what people expect to find. Thanks Sandro.

nixos/modules/config/nix-channel.nix

···
 
   See also
    - ./nix.nix
-   - ./flakes.nix
+   - ./nix-flakes.nix
  */
 { config, lib, ... }:
 let

nixos/modules/config/nix.nix

···
 
   See also
    - ./nix-channel.nix
-   - ./flakes.nix
+   - ./nix-flakes.nix
    - ./nix-remote-build.nix
    - nixos/modules/services/system/nix-daemon.nix
  */

nixos/modules/module-list.nix

···
   ./config/appstream.nix
   ./config/console.nix
   ./config/debug-info.nix
-  ./config/flakes.nix
   ./config/fonts/fontconfig.nix
   ./config/fonts/fontdir.nix
   ./config/fonts/fonts.nix
···
   ./config/networking.nix
   ./config/nix.nix
   ./config/nix-channel.nix
+  ./config/nix-flakes.nix
   ./config/nix-remote-build.nix
   ./config/no-x-libs.nix
   ./config/nsswitch.nix