Make more intelligent use of the cache

Bust cache on the dynamic files
Use the cached builds for both virtualbox and Hyper-V
Hopefully improve loop downloading files to cache

sources/virtualbox-iso.pkr.hcl

@@ -1,5 +1,5 @@
 source "virtualbox-iso" "amd64" {
-  iso_url          = var.iso.url
+  iso_urls         = [ "cache/${basename(var.iso.url)}", var.iso.url]
   iso_target_path  = "cache/${basename(var.iso.url)}"
   iso_checksum     = var.iso.checksum
   output_directory = "output/virtualbox/${local.name}"