ci: enable cache jobs on regular "main" pipelines

Scheduled pipelines on the "main" branch have cache jobs enabled, and
those jobs are used by more or less all pipelines in other repos for
quicker run.
Do this for ordinary "main" pipelines too, otherwise all jobs won't use
a cache for a week after merging anything to the builderv2 repo.

.gitlab-ci.yml

@@ -20,6 +20,8 @@ variables:
     description: "Run components jobs, default: 0"
   CI_RUN_TEMPLATE_JOBS:
     description: "Run template jobs, default: 0"
+  CI_RUN_CACHE_JOBS:
+    description: "Run cache jobs, subset of CI_RUN_COMPONENT_JOBS, default: 0"
   CI_RUN_ISO_JOBS:
     description: "Run ISO jobs, default: 0"
   CI_RUN_INFRA_JOBS:
@@ -75,7 +77,7 @@ builderv2-github:
 
 .init_cache_job:
   rules:
-    - if: $CI_RUN_CACHE_JOBS == "1" || $CI_RUN_COMPONENT_JOBS == "1" || $CI_RUN_ALL_JOBS == "1"
+    - if: $CI_RUN_CACHE_JOBS == "1" || $CI_RUN_COMPONENT_JOBS == "1" || $CI_RUN_ALL_JOBS == "1" || $CI_COMMIT_BRANCH == "main"
       when: always
     - when: never
   stage: prep