remove deprecated system passing

fixes
'system' has been renamed to/replaced by 'stdenv.hostPlatform.system'

Wherever possible, it's best to reference the hostPlatform of the
nixpkgs instance directly via `config`. This is only possible in NixOS
systems though, for standalon home-manager configs we need to get that
value through `pkgs.stdenv`.

flake.nix

@@ -80,14 +80,11 @@
         system: extraModules:
         nixpkgs.lib.nixosSystem rec {
           modules = (defaultModules system) ++ extraModules;
-          inherit system;
-          specialArgs = {
-            inherit system;
-          };
         };
       mkHomeManager =
         confName: user: system: # unfortunately, home-manager configs are still system-specific
         home-manager.lib.homeManagerConfiguration {
+          # XXX: when can we move to hostPlatform?
           pkgs = import (mkNixpkgs-patched system) { inherit system; };
           modules = [
             {
@@ -102,7 +99,7 @@
           # Optionally use extraSpecialArgs
           # to pass through arguments to home.nix
           extraSpecialArgs = {
-            inherit inputs system;
+            inherit inputs;
           };
         };
       treefmtConf =
@@ -200,7 +197,7 @@
               exit 0
             fi
             ${
-              inputs.home-manager.packages.${pkgs.system}.home-manager
+              inputs.home-manager.packages.${system}.home-manager
             }/bin/home-manager --flake "${self}#$profile" "$@"
           ''}/bin/hm";
         };