ci: Fix deps build on pushes to main
The YAML was configured to fetch a cache and use it whenever a cerbero pipeline is triggered via the GitLab pipeline trigger API, which happens in the "cerbero trigger" job in gstreamer/gstreamer, which runs gstreamer.git/ci/gitlab/trigger_cerbero_pipeline.py
But the job definition in Cerbero CI couldn't differentiate between a pipeline triggered that way and a pipeline triggered as post-merge CI to generate a Cerbero cache. The variable used to check that was CI_PIPELINE_SOURCE which is 'pipeline' in both those cases:
https://gitlab.freedesktop.org/gstreamer/cerbero/-/jobs/18446678 (post-merge CI) https://gitlab.freedesktop.org/gstreamer/cerbero/-/jobs/14644527 (triggered)
It is 'merge_request_event' when the pipeline is triggered by a Cerbero merge request:
https://gitlab.freedesktop.org/ystreet/cerbero/-/jobs/18433849
The only way to differentiate between the two is to add a custom trigger variable when triggering the pipeline from the gstreamer CI job: CI_GSTREAMER_TRIGGERED
We set it to "false" for now, which will cause all pipelines to to build from scratch. A corresponding MR on gstreamer/gstreamer will set it to "true" which will use the cache.