It appears to work if I get rid of all the JSON files except "ConsoleApp1.runtimeconfig.json" and then change that file to read:

{
  "runtimeOptions": {
    "tfm": "netcoreapp2.1",
    "framework": {
      "name": "Microsoft.NETCore.App",
      "version": "2.1.0"
    }
  }
}