Try pre-caching ISO files

Since the builds themselves get run in a big parallel morass of
nonsense, this causes cache clobbering all over. So now I move the
downloading to the pre-trigger stage and hopefully we can use
the pre-download step to cache things properly. Hopefully this will also
prevent issues where dynamically generated download files are missing
for later runs.

sources/vmware-iso.pkr.hcl

@@ -1,5 +1,6 @@
 source "vmware-iso" "amd64" {
   iso_url          = var.iso.url
+  iso_target_path  = "cache/${basename(var.iso.url)}"
   iso_checksum     = var.iso.checksum
   output_directory = "output/vmware/${local.name}"