chore(pm): replace pkgs.system uses

For fixing the warning

> trace: evaluation warning: 'system' has been renamed to/replaced by 'stdenv.hostPlatform.system'

packetmix/packages/josh/default.nix

···
   config.packages.josh = {
     systems = [ "x86_64-linux" ];
     package =
-      { system, rustPlatform, ... }:
-      config.inputs.nixos-unstable.result.${system}.josh.overrideAttrs {
+      { stdenv, rustPlatform, ... }:
+      config.inputs.nixos-unstable.result.${stdenv.hostPlatform.system}.josh.overrideAttrs {
         src = config.inputs.josh.result;
 
         cargoDeps = rustPlatform.fetchCargoVendor {

packetmix/packages/opensearch/default.nix

···
     systems = [ "x86_64-linux" ];
     package =
       {
-        system,
         opensearch,
         jdk11_headless,
         ...