feat: enable incus support in actionsdind container (#10)

- add incus-images backend
- set incus-images as default backend

Reviewed-on: https://git.query.consul/unkin/packer-images/pulls/10

library/locals.pkr.hcl

@@ -7,4 +7,6 @@ locals {
   incus_base_name = "${var.os_name}${var.os_version_major}/${var.os_image}"
   incus_output_image = "${local.incus_base_name}/${var.suffix}"
   is_master  = "${var.git_branch}" == "master"
+  incus_key = vault("kv/data/service/packer/builder/docker-incus-client", "private_key")
+  incus_crt = vault("kv/data/service/packer/builder/docker-incus-client", "public_key")
 }