Fix a couple few deprecations

[tjarratt]

I'm about 99.27% certain this pkgs.stdenv.hostPlatform.system syntax is correct. I tried to reproduce this locally with the flake you've provided but couldn't get the warning to appear locally when using nix develop and :lf . with nix repl.

However, I was able to reproduce using a separate flake that pulls this in as an input and using this command :

nix build .#homeConfigurations.\"$USER\".activationPackage \
  --option abort-on-warn true \
  --option show-trace true

https://discourse.nixos.org/t/how-to-fix-evaluation-warning-system-has-been-renamed-to-replaced-by-stdenv-hostplatform-system/72120

nix-community/home-manager@ac7c05a