Change pkgs.system to pkgs.stdenv.hostPlatform.system in cardano-node-service.nix

[zeme-wana]

Description

This fixes the following evaluation warning on recent nixpkgs releases:

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

We use the cardano-node-service.nix module in our NixOS config like so:

  imports = [
    ./ax41-nvme.nix
    ./common.nix
    "${inputs.cardano-node}/nix/nixos/cardano-node-service.nix"
  ];

This PR makes the warning disappear.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Runnings tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff