CI: don't deploy docs from scheduled pipelines

The package auto-update runs in a scheduled pipeline, it should be
the only job that runs however a discrepency in the rules for the
deploy-docs job results in it running as well.

To mitigate this, add an additional rule to never run the job in a
scheduled pipeline.

Signed-off-by: Casey Connolly <kcxt@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7457>

.gitlab-ci.yml

@@ -219,6 +219,8 @@ deploy-docs:
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH && $CI_PROJECT_NAMESPACE == "postmarketOS"
       changes:
         - docs/**
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+      when: never
 
 auto-update:
   stage: autoupdate