Update cyg-get

Update cyg-get to be able to run straight after installing cyg-get, if you don't close and reopen Powershell after installing cyg-get (spawning a new Powershell within doesn't work), cyg-get doesn't work. This will make cyg-get find the cygwinsetup.exe without restarting Powershell.
It won't be needed as $cygRoot gets the cygwin root (which will be the working directory after the cd) and specifies the cygwin root when running cygwinsetup.exe. I mean we could just set $cygRoot from that reg value instead, then get $cygwinsetup from that to simplify the code very slightly. But I don't think it's really needed at all.