nixos/manual: enhance appimage instructions (#350793)

Most pre-built executables will not work on NixOS. There are two notable

exceptions: flatpaks and AppImages. For flatpaks see the [dedicated

section](#module-services-flatpak). AppImages will not run "as-is" on NixOS.

First you need to install `appimage-run`: add to `/etc/nixos/configuration.nix`

section](#module-services-flatpak). AppImages can run "as-is" on NixOS.

First you need to enable AppImage support: add to `/etc/nixos/configuration.nix`

```nix

{

  environment.systemPackages = [ pkgs.appimage-run ];

  programs.appimage.enable = true;

  programs.appimage.binfmt = true;

}

```

Then instead of running the AppImage "as-is", run `appimage-run foo.appimage`.

Then you can run the AppImage "as-is" or with `appimage-run foo.appimage`.

If there are shared libraries missing add them with

```nix

{

  programs.appimage.package = pkgs.appimage-run.override {

    extraPkgs = pkgs: [

      # missing libraries here, e.g.: `pkgs.libepoxy`

    ];

  }

}

```

To make other pre-built executables work on NixOS, you need to package them

with Nix and special helpers like `autoPatchelfHook` or `buildFHSEnv`. See