light: Add meta.mainProgram

Add `meta.mainProgram` for use with `lib.getExe`.

pkgs/os-specific/linux/light/default.nix

@@ -35,6 +35,7 @@ stdenv.mkDerivation rec {
     description = "GNU/Linux application to control backlights";
     homepage = "https://haikarainen.github.io/light/";
     license = lib.licenses.gpl3;
+    mainProgram = "light";
     maintainers = with lib.maintainers; [ puffnfresh dtzWill ];
     platforms = lib.platforms.linux;
   };