Try adding <RunWixToolsOutOfProc>true</RunWixToolsOutOfProc> to your WiX project file.
We’ve had the same issue for a while, and tried various workarounds including deleting the temporary files and setting the msbuild environment variable. These all appeared to work for a while, but eventually (sometimes after a few days) the problem would come back again.
I noticed that on my machine devenv.exe was the process that was locking the files that light.exe was trying to delete. I also stumbled across an unrelated thread which mentioned this project setting to make the WiX tools run out of process. I thought it could be worth a try and it appears to have cured the problem for us (so far…)