Accept Chef license on first use in AppVeyor pipeline

Otherwise the pipeline fails on this command with the following error
message:

> Chef Development Kit cannot execute without accepting the license

It seems the command-line option to accept the licens can't be placed
such that it isn't erroneously interpreted as a subcommand (whether
before or after the exec subcommand), so I went with the environment
variable instead, which does seem to work correctly.

appveyor.yml

@@ -18,6 +18,7 @@ install:
   - ps: iex (irm https://omnitruck.chef.io/install.ps1); Install-Project -Project chefdk -channel current
   - ps: 'Get-CimInstance win32_operatingsystem -Property Caption, OSArchitecture, Version | fl Caption, OSArchitecture, Version'
   - ps: $PSVersionTable
+  - ps: '$env:CHEF_LICENSE = "accept"'
   - c:\opscode\chefdk\bin\chef.bat exec ruby --version
   - ps: secedit /export /cfg $env:temp/export.cfg
   - ps: ((get-content $env:temp/export.cfg) -replace ('PasswordComplexity = 1', 'PasswordComplexity = 0')) | Out-File $env:temp/export.cfg