[ naturallaw777/Sovran_SystemsOS ] Fix btcpayserver syntax error and pin to nixpkgs-stable (2.4.2)

1. flake.nix: Add pkgs-stable as specialArg for modules
2. btcpayserver.nix:
- Fix preStart script syntax error (closing brace on new line)
- Use pkgs-stable.btcpayserver (2.4.2) instead of pkgs.btcpayserver
- Use pkgs-stable.nbxplorer (2.6.10) instead of pkgs.nbxplorer
- Remove clightning references (Sovran is LND-only)
https://github.com/naturallaw777/Sovran_SystemsOS/commit/2c6ef7b60e94bf87be8ef4f8c67507520ef940c6