That won’t work - if you run it from a cmd prompt it will only affect that process and its child processes. (So if you start Visual Studio from a desktop icon, it won’t be affected.)
Instead go to System Properties, Advanced, and click Environment Variables. Put the variable and value there and press OK. Confirm it works by starting a NEW cmd session and typing SET AUTO and you should see AUTOUPDATER_Duplicati_SKIP_UPDATE=1 listed.
Then start VS and try again. 
Good luck!