appveyor.yml: add APPVEYOR_BUILD_SYS
appveyor.yml: add APPVEYOR_BUILD_SYS

bryson/packer,mitchellh/packer,paulmey/packer,mkuzmin/packer,bryson/packer,mitchellh/packer,dave2/packer,youhong316/packer,pecigonzalo/packer,dave2/packer,mitchellh/packer,rnaveiras/packer,pecigonzalo/packer,mkuzmin/packer,ricardclau/packer,ricardclau/packer,bryson/packer,dave2/packer,rnaveiras/packer,grubernaut/packer,paulmey/packer,pecigonzalo/packer,dave2/packer,tb3088/packer,ricardclau/packer,bryson/packer,tb3088/packer,rnaveiras/packer,timsutton/packer,grubernaut/packer,mitchellh/packer,dave2/packer,grubernaut/packer,tas50/packer,dave2/packer,tas50/packer,paulmey/packer,youhong316/packer,grubernaut/packer,timsutton/packer,hashicorp/packer,youhong316/packer,ricardclau/packer,mkuzmin/packer,mkuzmin/packer,grubernaut/packer,rnaveiras/packer,hashicorp/packer,youhong316/packer,ricardclau/packer,rnaveiras/packer,mkuzmin/packer,hashicorp/packer,tas50/packer,tb3088/packer,youhong316/packer,paulmey/packer,timsutton/packer,tas50/packer,bryson/packer,bryson/packer,timsutton/packer,tb3088/packer,tas50/packer,ricardclau/packer,paulmey/packer,tas50/packer,rnaveiras/packer,grubernaut/packer,timsutton/packer,paulmey/packer,timsutton/packer,tb3088/packer,mitchellh/packer,tb3088/packer,youhong316/packer,hashicorp/packer,pecigonzalo/packer,mkuzmin/packer