chore: change snapshot detection to prefix detection

If the provided checksum does not have a prefix, we assume it is user-provided,
and should be treated as a snapshot.

openbsd.pkr.hcl

@@ -109,14 +109,15 @@ variable "readonly_boot_media" {
 locals {
   image_architecture = var.architecture == "x86-64" ? "amd64" : var.architecture
   image = "miniroot${replace(var.os_version, ".", "")}.img"
+  has_checksum_prefix = substr(var.checksum, 0, 7) == "sha256:"
 
   image_full_remote_path = (
-    length(var.checksum) != 0
-      ? "snapshots/${local.image_architecture}/${local.image}"
-      : "${var.os_version}/${local.image_architecture}/${local.image}"
+    has_checksum_prefix
+      ? "${var.os_version}/${local.image_architecture}/${local.image}"
+      : "snapshots/${local.image_architecture}/${local.image}"
   )
 
-  checksum = substr(var.checksum, 0, 7) == "sha256:" ? var.checksum : join(":", ["sha256", var.checksum])
+  checksum = has_checksum_prefix ? var.checksum : join(":", ["sha256", var.checksum])
   vm_name = "openbsd-${var.os_version}-${var.architecture}.qcow2"
 
   iso_target_extension = "img"