nixos/release-notes: explain the new system.nix file

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- The system.nix file has been added has an alternative entry point to configuration.nix (and flake.nix) that allows to configure NixOS without using `nix-channel`.

  This file must evaluate to a NixOS system derivation or an attribute set of such derivations, in which case the attribute to build has to be selected with the `--attr` option of `nixos-rebuild` or `nixos-install`.

  For example,

  ```nix

  # system.nix

  let

    # Pinned Nixpkgs archive

    #

    # Use `curl -I https://channels.nixos.org/nixos-26.05` to get the

    # latest commit of the stable channel and `nix-prefetch-url --unpack`

    # to compute its sha256 hash.

    nixpkgs = builtins.fetchTarball {

      url = "https://github.com/NixOS/nixpkgs/archive/c217913993d6.tar.gz";

      sha256 = "026mprs324330pfazlgbw987qmsa8ligglarvqbcxzig2kgw0lqg";

    };

  in

  import "${nixpkgs}/nixos" {

    # Build NixOS using an external configuration.nix file,

    # or directly set your options here

    configuration = ./configuration.nix;

  }

  ```

  The default location of system.nix is `/etc/nixos/system.nix` and can be changed by setting the `<nixos-system>` search path.

  `nixos-rebuild` and `nixos-install` can now also load a system.nix file in the current directory (only if `--attr` is used) or from a directory specified with `--file`.

- The default kernel package has been updated from 6.12 to 6.18. All supported kernels remain available.

## New Modules {#sec-release-26.05-new-modules}