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)