Motivation for this change

Hercules CI is a continuous integration system that uses Nix as its foundation. Users run hercules-ci-agent themselves to execute their CI jobs. Packaging it in Nixpkgs will make installation easier. A NixOS module will be provided in a separate PR.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date -> meta fields are up to date and complete compared to pkgs.nix
• Fits CONTRIBUTING.md.