Move almost everything over to Nix

Eliminate Xonsh usage
Eliminate usage of the supports.yml files for default.nix
Point ISOs directly to MinIO URLs instead

sources/inputs.pkr.hcl

@@ -109,6 +109,16 @@ variable "vbox_os_type" {
   default = "Linux_64"
 }
 
+variable "storage_url" {
+  type    = string
+  default = env("STORAGE_URL")
+}
+
+variable "minio_secret" {
+  type    = string
+  default = env("MINIO_SECRET")
+}
+
 locals {
   name = "${var.distro}-${var.version}"
   description = templatefile("../README.box.md", {