scripts/build.sh: allow to set build settings when building
scripts/build.sh: allow to set build settings when building

this will for example allow me to have the following alias:
alias buildmain='export T=$(mktemp -d) && ALL_XC_OS="linux darwin windows" ALL_XC_ARCH="amd64" GOLDFLAGS="-s -w" ./scripts/build.sh && cd pkg/ && for dir in *; do zip -r "$dir.zip" $dir & ; done ;  wait && mv *.zip $T/. && open $T'

that build only on 'main' platforms, so that I can share binaries easily.

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