Fix zsh auto-completion
Fix zsh auto-completion

Closes #8158

This fixes the following errors when attempting to auto-complete with packer v.1.4.3:
(installed via Homebrew on macOS Mojave)

 `invalid option definition: -except=[(foo,bar,baz) Validate all builds other than these].`
 `invalid option definition: -only=[(foo,bar,baz) Validate only these builds].`

pecigonzalo/packer,paulmey/packer,timsutton/packer,grubernaut/packer,rnaveiras/packer,paulmey/packer,rnaveiras/packer,timsutton/packer,pecigonzalo/packer,grubernaut/packer,grubernaut/packer,hashicorp/packer,grubernaut/packer,hashicorp/packer,paulmey/packer,mitchellh/packer,pecigonzalo/packer,pecigonzalo/packer,timsutton/packer,hashicorp/packer,ricardclau/packer,ricardclau/packer,grubernaut/packer,paulmey/packer,mitchellh/packer,rnaveiras/packer,ricardclau/packer,dave2/packer,dave2/packer,hashicorp/packer,paulmey/packer,timsutton/packer,mitchellh/packer,ricardclau/packer,ricardclau/packer,mitchellh/packer,dave2/packer,ricardclau/packer,rnaveiras/packer,dave2/packer,mitchellh/packer,timsutton/packer,dave2/packer,dave2/packer,rnaveiras/packer,rnaveiras/packer,paulmey/packer,timsutton/packer,grubernaut/packer