Recurring Error/Failure downloading dotnet core, download path unavailable?

See https://github.com/microsoft/vscode-azurearmtools/issues/1062

> After further review, I have found the following:
>
> The dotnet-install.ps1 script is trying to download the dotnet binaries from this URL: https://dotnetcli.azureedge.net/dotnet/Runtime/3.1.9/dotnet-win-x64.3.1.9.zip, based on a query to some kind of Azure "Feed". That path is not reachable so it continuously fails. I modified the script to point it to a local path of that zip, and it does install. The issue then is that when VS Code starts, it wipes out the binaries that were copied into place using the script. So it removes the files from where it needs them.
>
> From that point forward, VS Code continually fails to download the files. To workaround this error, I used the static path under "dotnetAcquisitionExtension.existingDotnetPath" to set the path to my manually installed dotnet core install.
>
> The root issue seems to be the install script is trying to download the zip file from a URL that is not available.