ci: Fix project URL when triggering cerbero pipelines

ci: Fix project URL when triggering cerbero pipelines

When merge request pipelines are triggered on the gstreamer namespace,
CI_PROJECT_URL will be gitlab.[...]/gstreamer/gstreamer but we need to
use gitlab.[...]/$USER/gstreamer because that's where the source
branch is located.

This exhibits as cerbero pipelines failing because it can't find the
specified branch:

https://gitlab.freedesktop.org/gstreamer/cerbero/-/pipelines/639379

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2761>