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.

.gitlab-ci-build.yml.in

@@ -11,8 +11,9 @@ stages:
           - sources/build.pkr.hcl
       paths:
         - ./packer_config/
-    - paths:
-        - packer_cache/*.iso
+    - key: isofiles
+      paths:
+        - cache/*.iso
   rules:
     - if: $CI_COMMIT_TAG
       variables:
@@ -31,7 +32,7 @@ stages:
     - ${tag}
   variables: &variables
     PACKER_CONFIG_DIR: ./packer_config/
-    PACKER_CACHE_DIR: ./packer_cache/
+    PACKER_CACHE_DIR: ./cache/
     PACKER_LOG: 1
     COMMAND: >
       packer build ${only}