* Change `ping .. > nul` to `sleep`
The filename `nul` is a Windows-ism that causes the update script to
create such a file in the current working directory. The `ping`
utility is also an dependency not installed by
`installdependencies.sh`, so it seemed easier to change it to the
standard `sleep` command.
* Update dotnet-install script as requested by test
* Update dotnet-install.ps1
Co-authored-by: Tingluo Huang <tingluohuang@github.com>
