ci: Allow to run scheduled pipelines

While everything that's merged needs to have passed CI, the
testsuite result are not as consistent as we'd like and sometimes
regress.

It's nice to have have a pipeline hitory of the tests results
to make it easier to spot regressions that might have caused
racy/flaky tests.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5588>

.gitlab-ci.yml

@@ -67,6 +67,9 @@ variables:
 workflow:
   # https://docs.gitlab.com/ee/ci/yaml/index.html#switch-between-branch-pipelines-and-merge-request-pipelines
   rules:
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+      variables:
+        GIT_FETCH_EXTRA_FLAGS: '--no-tags'
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
       variables:
         GIT_FETCH_EXTRA_FLAGS: '--no-tags'
@@ -102,6 +105,8 @@ trigger:
   script:
     - echo "Trigger job done, now running the pipeline."
   rules:
+    # If this pipeline is triggered by a schedule, run automatically
+    - if: $CI_PIPELINE_SOURCE == "schedule"
     # If the MR is assigned to the Merge bot, trigger the pipeline automatically
     - if: '$CI_MERGE_REQUEST_ASSIGNEES == "gstreamer-merge-bot"'
     # Require explicit action to trigger tests post merge, but we want to