remkrom: add meta.mainProgram

pkgs/development/tools/knightos/mkrom/default.nix

···
     license     = licenses.mit;
     maintainers = with maintainers; [ siraben ];
     platforms   = platforms.all;
-    mainProgram = "remkrom";
   };
 }

pkgs/development/tools/knightos/remkrom/default.nix

···
     homepage = "https://github.com/siraben/remkrom";
     license = licenses.mit;
     maintainers = with maintainers; [ siraben ];
+    mainProgram = "remkrom";
   };
 }