fix: replace pkgs.system with pkgs.stdenv.hostPlaform.system

[toyboot4e]

Sources:

• https://www.github.com/snowfallorg/lib/issues/140
• How to fix evaluation warning: ‘system’ has been renamed to/replaced by ‘stdenv.hostPlatform.system’

[toyboot4e]

I think this change is trivial enough to be merged without review.

What I tested:

• nix build .#plover-full
• The README setting with my branch:

    package = inputs.plover-flake.packages.${pkgs.stdenv.hostPlatform.system}.plover.withPlugins (
      ps: with ps; [
        plover-lapwing-aio
      ]
    );