Add NuGet source

Author: ronaldbarendse

.github/workflows/publish.yml

@@ -35,4 +35,4 @@ jobs:
         run: umbpack push ${{ steps.get_package.outputs.PACKAGE }} -k ${{ secrets.OURUMBRACO_API_KEY }} -a *
 
       - name: Push to NuGet
-        run: dotnet nuget push *.nupkg --api-key ${{ secrets.NUGET_API_KEY }} --skip-duplicate
+        run: dotnet nuget push *.nupkg --source https://api.nuget.org/v3/index.json --api-key ${{ secrets.NUGET_API_KEY }} --skip-duplicate