v4l2loopback: Fix issue with buildEnv using bin output

See https://github.com/NixOS/nixpkgs/pull/134311#issuecomment-901952544

pkgs/os-specific/linux/v4l2loopback/default.nix

···
     license = licenses.gpl2Only;
     maintainers = with maintainers; [ fortuneteller2k ];
     platforms = platforms.linux;
+    outputsToInstall = [ "out" ];
   };
 }