ci: Switch from URL to PATH for trigger CI variable

Use a path, which contains the namespace and the repo name instead of the complete URL when triggering the Cerbero CI pipeline. Some of the jobs in the Cerbero pipeline (specifically the examples build) need to curl the API, and they need the path for that, not the website or git URL.

The examples build doesn't use gst-plugins-rs, but we change this at the same time as the monorepo for consistency.