The energy comes from the chemical energy stored in the muscles of your arms. But ...
You have to be careful when you say that net work is zero. You have to make sure that your system is clearly defined so that you know what's in your system, and what's outside. Then you have to identify what work is internal, and what work is external, and which if any is zero.
One way of defining the system in your case is to say that the Earth and bucket are inside the system, everything else is outside. Then the rope (not you) does work on the system. This work is external work ... it come into the system from the outside.
So the work done on the system in this picture is not zero.
You can also analyze a system comprising the Earth, the bucket, the rope, and you. This case is more complicated because 1.) you now have an energy storage device (you) in the system, and 2.) you are deformable and 3.) The force produced by your muscles is not conservative. Nonetheless, in this case the total internal work is indeed zero. The energy comes from chemical energy in your muscles.