Move dotnet-install script source to the URL they redirect to, to hopefully bypass SSL Cert mismatch errors

[MattGal]

Try to sort https://github.com/dotnet/core-eng/issues/11391 by using the non-redirected URL (the old path actually is a redirect to this URL)

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation (this is covered by the PR builds)