update-luarocks: Several improvements
update-luarocks: Several improvements

Changes:
- Fetches rocks and builds Nix expressions for them in parallel
- Passes 'maintainers' list to luarocks-nix
- Constructs the luarocks argument list more cleanly, by using an
  indexed array
- Made indentation consistent

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs