fix warning system -> stdenv.hostPlatform.system

[pineapplehunter]

Recently using the value of pkgs.system got deprecated and an warning appeares when I try to use the overlay.
This updates it to stdenv.hostPlatform.system.

[willbush]

here's a repro if anyone was wanting:

nix eval --impure --option eval-cache false --expr '
    let
      nixpkgs = builtins.getFlake "github:NixOS/nixpkgs/nixos-unstable";
      f = builtins.getFlake (toString ./.);
      pkgs = import nixpkgs {
        system = "x86_64-linux";
        overlays = [ f.overlay ];
      };
    in pkgs.nixgl.nixVulkanIntel.drvPath
    '

It starting emitting new warnings for xorg package set being deprecated. Think these warnings don't exist on the current pinned version of nixpkgs that nixgl is using.

evaluation warning: 'system' has been renamed to/replaced by 'stdenv.hostPlatform.system'
evaluation warning: The xorg package set has been deprecated, 'xorg.libX11' has been renamed to 'libx11'
evaluation warning: The xorg package set has been deprecated, 'xorg.libxcb' has been renamed to 'libxcb'
evaluation warning: The xorg package set has been deprecated, 'xorg.libxshmfence' has been renamed to 'libxshmfence'
"/nix/store/vplqdqw3aapcpidwfy8ndwbsjp4qb1qg-nixVulkanIntel.drv"

I'm only seeing these because I'm doing:

    nixgl = {
      url = "github:nix-community/nixGL";
      inputs.nixpkgs.follows = "nixpkgs";
    };

in my flake.nix.