scripts/build.sh: don't discard stderr when copying packer
scripts/build.sh: don't discard stderr when copying packer

fix #7604
ricardclau/packer,rnaveiras/packer,grubernaut/packer,dave2/packer,dave2/packer,paulmey/packer,hashicorp/packer,pecigonzalo/packer,tas50/packer,timsutton/packer,ricardclau/packer,grubernaut/packer,dave2/packer,mitchellh/packer,grubernaut/packer,pecigonzalo/packer,paulmey/packer,bryson/packer,rnaveiras/packer,hashicorp/packer,bryson/packer,hashicorp/packer,hashicorp/packer,grubernaut/packer,mitchellh/packer,timsutton/packer,timsutton/packer,dave2/packer,tas50/packer,tas50/packer,dave2/packer,pecigonzalo/packer,rnaveiras/packer,paulmey/packer,paulmey/packer,paulmey/packer,bryson/packer,grubernaut/packer,bryson/packer,timsutton/packer,grubernaut/packer,ricardclau/packer,tas50/packer,rnaveiras/packer,ricardclau/packer,timsutton/packer,tas50/packer,bryson/packer,rnaveiras/packer,ricardclau/packer,mitchellh/packer,tas50/packer,pecigonzalo/packer,ricardclau/packer,mitchellh/packer,bryson/packer,dave2/packer,paulmey/packer,timsutton/packer,rnaveiras/packer,mitchellh/packer