Minor: Get-ChefDK fails if current drive is not writable

[chefsalim]

Noticed this during debugging.
If install-workstation is called when local drive is not writable:, it fails with the following message:
In this case, current drive is Z: which is not writable.

Cloning https://github.com/chef/pantry-chef-repo to C:\Users\salim\Documents\pantry-chef-repo ...
Downloading ChefDK
Get-ChefDK : System.UnauthorizedAccessException: Access is denied. (Exception from HRESULT: 0x80070005
(E_ACCESSDENIED))
at Microsoft.BackgroundIntelligentTransfer.Management.NewBitsTransferCommand.ProcessRecord()
at System.Management.Automation.CommandProcessor.ProcessRecord()
At C:\Users\salim\Documents\pantry-chef-repo\bin\pantry.ps1:80 char:3

• Get-ChefDK
• CategoryInfo : NotSpecified: (:) [Write-Error], WriteErrorException
• FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,Get-ChefDK

PS Z:>