dotnet-install does not handle redirects when downloading the product version files

[mmitche]

When downloading the product version files, the curl command doesn't follow redirects. We've seen this with a custom feed. In this case, get_specific_product_version returns an empty string. The script also erroneously succeeds when this happens. The specific version ends up empty, and then the check for whether the version already exists succeeds. It finds <install location>/shared/Microsoft.NETCore.App/ (empty version directory).