the switching power supply in the light is creating radio noise on the same frequency that the speedometer transmitter uses. Shielding the speedometer receiver is just going to reduce the signal strength from the transmitter, so if you want to add shielding, it needs to be added to the light.

Adding shielding to the light needs to be done in the right way. It should be done at the circuit itself and connected to the battery negative. However, it needs to be done at the circuit board.
The noise can also be radiated from any wires leaving the circuit board, so a similar shield should be added around these wires.

I do this sort of thing for a living, and it's rarely a simple solution. There are no regulations for this sort of problem, so the manufacturers of lights and bike computers have no guidelines to design to. I'd suggest contacting the light manufacturer and see if they know that their light is causing problems and see if they can suggest a fix. Perhaps they can even swap it for a part that doesn't emit so much noise??

The alternatives have already been discussed.. moving the light away from the bike computer or switching to a wired bike computer... or moving the computer's transmitter closer to the receiver.