gammastep: set meta.mainProgram

pkgs/applications/misc/redshift/default.nix

···
       longDescription = "Gammastep"
         + lib.removePrefix "Redshift" redshift.meta.longDescription;
       homepage = "https://gitlab.com/chinstrap/gammastep";
+      mainProgram = "gammastep";
       maintainers = [ lib.maintainers.primeos ] ++ redshift.meta.maintainers;
     };
   };