fixup: Use environment variable to accept Chef license

It seems the command-line option can't be placed such that it isn't
interpreted as a subcommand instead of an option (whether before or
after the exec subcommand), so gotta try with the environment variable
instead.

appveyor.yml

@@ -18,7 +18,8 @@ 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: $PSVersionTable
-  - c:\opscode\chefdk\bin\chef.bat exec --chef-license accept ruby --version
+  - ps: '$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
   - ps: ((get-content $env:temp/export.cfg) -replace ('MinimumPasswordLength = 8', 'MinimumPasswordLength = 0')) | Out-File $env:temp/export.cfg