Windows PowerShell

I can't figure out what commit did this, but the OS detection routine in installpsh-amazonlinux.sh has been dramatically simplified from what is still in install-powershell.sh.

It is now missing the amazon linux detection routines.

The original design allowed for the underlying installers to be called directly - but they still need the full linux detection routine in order validate they are being run on the correct OS.

My container builds that install PowerShell are now failing:

https://gitlab.com/DarwinJS/amazon-linux-2-container-with-powershell/-/jobs/189478956
https://gitlab.com/DarwinJS/amazon-linux-2-ec2-build-container-with-powershell/-/jobs/189478955

For the time being I'm going to switch my build to a known working branch in my own fork of this repo.

Is there a reason this does not show up in automated testing?

I don't personally have the time to fix this.

/cc @TravisEz13