packer: Fetch debian10 image from shacache

Only shacache assures stable URLs for reproducible builds.

packer/debian10.json

@@ -20,8 +20,8 @@
       "format": "qcow2",
       "headless": true,
       "http_directory": "http",
-      "iso_checksum": "md5:36de671429939e90f2a31ce3fbed0aaf",
-      "iso_url": "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-10.2.0-amd64-netinst.iso",
+      "iso_checksum": "md5:a3ebc76aec372808ad80000108a2593a",
+      "iso_url": "https://shacache.nxdcdn.com/0a6aee1d9aafc1ed095105c052f9fdd65ed00ea9274188c9cd0072c8e6838ab40e246d45a1e6956d74ef1b04a1fc042151762f25412e9ff0cbf49418eef7992e",
       "name": "debian10-{{ user `disk_size`}}G-{{ user `name`}}",
       "shutdown_command": "echo '{{user `password`}}'|sudo -S shutdown -h now",
       "ssh_password": "{{user `password`}}",