ci-package-builder: Skip pull if DEBIAN_MIRROR is not set

Do not run the pull-updates job if $DEBIAN_MIRROR is not set. This is going to be useful to downstream projects which mirror the Apertis repositories and do not need to pull from Debian directly.

ci-package-builder.yml

@@ -89,7 +89,10 @@ pull-updates:
     - lightweight
   resource_group: $CI_PROJECT_ID
   only:
-    - /^debian\//
+    refs:
+      - /^debian\//
+    variables:
+      - $DEBIAN_MIRROR
   script:
     - PACKAGE=$(dpkg-parsechangelog -SSource || echo "$CI_PROJECT_NAME")
     - >