installation: remove NixOS

Installation.md

@@ -127,14 +127,6 @@ Check the [Streamlink install documentation][streamlink-install] for a detailed
 
     A mirror repo with branches of all the AUR packages can be found [here][aur-mirror-repo] where issues and pull requests can be submitted.
 
-  - #### NixOS
-
-    [`streamlink-twitch-gui-bin`][nixos-streamlink-twitch-gui-bin]
-
-    ```bash
-    nix-env -iA nixos.streamlink-twitch-gui-bin
-    ```
-
   - #### Solus (EOPKG)
 
     [`streamlink-twitch-gui`][eopkg-streamlink-twitch-gui]
@@ -174,7 +166,6 @@ Check the [Streamlink install documentation][streamlink-install] for a detailed
 [aur-streamlink-twitch-gui-git]: https://aur.archlinux.org/packages/streamlink-twitch-gui-git "Streamlink Twitch GUI AUR development package"
 [aur-streamlink-twitch-gui]: https://aur.archlinux.org/packages/streamlink-twitch-gui "Streamlink Twitch GUI AUR source package"
 [aur-mirror-repo]: https://github.com/streamlink/streamlink-twitch-gui-aur "Streamlink Twitch GUI AUR mirror repo"
-[nixos-streamlink-twitch-gui-bin]: https://search.nixos.org/packages?show=streamlink-twitch-gui-bin&query=streamlink "Streamlink Twitch GUI NixOS binary package"
 [eopkg-streamlink-twitch-gui]: https://github.com/getsolus/packages/tree/main/packages/s/streamlink-twitch-gui "Streamlink Twitch GUI Solus package"
 [custom-package-repo-experiment]: https://github.com/streamlink/streamlink-twitch-gui/pull/319
 [appimagelauncher]: https://github.com/TheAssassin/AppImageLauncher