Externalize cache file

cache.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -eo pipefail
+
+files=()
+
+for vars in $(find distros/ -name '*.pkrvars.hcl'); do
+  file=$(echo var.iso.url | packer console -var-file=${vars} -config-type=hcl2 sources/)
+  base="$(basename "${file}")"
+  if [ -e "cache/${base}" ]; then
+    echo "Found cache/${base} already"
+  else
+    files+=("-O" "${file}")
+  fi
+done
+
+mkdir -p cache
+pushd cache
+echo "Downloading ${files[@]}"
+curl --parallel -L "${files[@]}"
+popd