Pre-existing VAGRANT_DETECTED_OS env setting causes installation to fail

Having a pre-existing VAGRANT_DETECTED_OS env setting (for example VAGRANT_DETECTED_OS = cygwin), causes the installer to fail to install the CDK.

The error written to the install log is:

Mon, 06 Jun 2016 16:00:03 GMT-ERROR: cdk - Error: Command failed: vagrant box add --name cdkv2 "c:\DevelopmentSuite\cdk\boxes\rhel-vagrant-virtualbox.box"
[0;31mThe executable 'cygpath' Vagrant is trying to run was not
found in the %PATH% variable. This is an error. Please verify
this software is installed and on the path. [0m

Mon, 06 Jun 2016 16:00:03 GMT-ERROR: cdk - [0;31mThe executable 'cygpath' Vagrant is trying to run was not
found in the %PATH% variable. This is an error. Please verify
this software is installed and on the path. [0m

Mon, 06 Jun 2016 16:00:03 GMT-ERROR: cdk failed to install: Error: Command failed: vagrant box add --name cdkv2 "c:\DevelopmentSuite\cdk\boxes\rhel-vagrant-virtualbox.box"
[0;31mThe executable 'cygpath' Vagrant is trying to run was not
found in the %PATH% variable. This is an error. Please verify
this software is installed and on the path. [0m