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/qemu.pkr.hcl

@@ -4,7 +4,7 @@ variable "qemu_accelerator" {
 }
 
 source "qemu" "amd64" {
-  iso_urls         = [ "cache/${basename(var.iso.url)}" ]
+  iso_urls         = [ "s3::${var.storage_url}isos/cache/${basename(var.iso.url)}?aws_access_key_id=root&aws_access_key_secret=${var.minio_secret}" ]
   iso_target_path  = "cache/${basename(var.iso.url)}"
   iso_checksum     = var.iso.checksum
   output_directory = "output/libvirt/${local.name}"