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.

.gitignore

@@ -18,6 +18,7 @@ __pycache__
 *.pyc
 *.pyo
 packer_cache
+cache
 .venv
 venv