darkman: add meta.mainProgram

[bobvanderlinden]

Description of changes

Adds meta.mainProgram for darkman.
Motivation: home-manager uses getExe darkman, which results in warnings during builds.

With this change the warnings doesn't show up:

$ nix repl
Welcome to Nix 2.17.0. Type :? for help.

nix-repl> :l .
Added 19492 variables.

nix-repl> lib.getExe darkman 
"/nix/store/jzjs8b4rj6hfxvvf320ac9dw3z6ab8wb-darkman-1.5.4/bin/darkman"

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.