ci: fix nightly build (v2)

Scheduled pipelines don't have `changes`, so the rule I added in
fa4dd11098 won't match; re-do that but as its own rule,
without the additional `changes` condition.

Fixes: 7c0b19a607 ("ci: run python-test automatically only in merge pipelines")
Fixes: fa4dd11098 ("ci: fix nightly build")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28561>

.gitlab-ci/test/gitlab-ci.yml

@@ -59,7 +59,9 @@ python-test:
   rules:
     - !reference [.disable-farm-mr-rules, rules]
     - !reference [.never-post-merge-rules, rules]
-    - if: '$GITLAB_USER_LOGIN == "marge-bot" || $CI_PIPELINE_SOURCE == "schedule"'
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+      when: on_success
+    - if: $GITLAB_USER_LOGIN == "marge-bot"
       changes: &bin_ci_files
         - .gitlab-ci.yml
         - .gitlab-ci/**/*