downloads: pre-filter the loop instead of using when (#13563)

This prevent evaluation of the whole item even if not used, and the
tasks scheduling of skipped tasks.
Preventing evaluation in particular helps avoiding irrelevant errors,
like when an unused downloads member has undefined members.

roles/download/tasks/main.yml

@@ -18,13 +18,15 @@
 
 - name: Download | Download files / images
   include_tasks: "{{ include_file }}"
-  loop: "{{ downloads | combine(kubeadm_images) | dict2items }}"
+  loop: "{{ [] if skip_downloads
+            else (
+              (downloads | combine(kubeadm_images)).values()
+              | selectattr('enabled')
+            )
+          }}"
   vars:
-    download: "{{ download_defaults | combine(item.value) }}"
+    download: "{{ download_defaults | combine(item) }}"
     include_file: "download_{% if download.container %}container{% else %}file{% endif %}.yml"
   when:
-    - not skip_downloads
-    - download.enabled
-    - item.value.enabled
     - (not download.container) or (download.container and download_container)
     - (download_run_once and inventory_hostname == download_delegate) or (group_names | intersect(download.groups) != [])