Switch to using CDN for install script

[richlander]

We're relying on the install script being delivered via the website not the CDN. That needs to be changed, in both the .ps1 and .sh files

https://github.com/dotnet/arcade/blob/f2135575461b9adce73a7178f0a9692c2b8608f1/eng/common/tools.sh#L298

There are also multiple references to the website.

https://github.com/dotnet/arcade/blob/f2135575461b9adce73a7178f0a9692c2b8608f1/eng/common/tools.sh#L310-L311

and

https://github.com/dotnet/arcade/blob/f2135575461b9adce73a7178f0a9692c2b8608f1/eng/common/tools.sh#L56-L58

I see that the install script is actually in the same directory. Why not just use it?

https://github.com/dotnet/arcade/blob/main/eng/common/dotnet-install.sh