Improve cache usage

1. Only download cached files that are not present in the current cache
2. Try pulling directly from the cache, not only with the target
   information

sources/qemu.pkr.hcl

@@ -4,7 +4,7 @@ variable "qemu_accelerator" {
 }
 
 source "qemu" "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/libvirt/${local.name}"