ci: Clone the right gstreamer repo/branch when triggered

In e1edfb10, I accidentally broke cloning of the user's gstreamer fork when cerbero CI was triggered inside a gstreamer monorepo MR. Fix that, and rework how we select the gstreamer and gst-plugins-rs remotes and commits in the various cases applicable in CI today.

Also move the android and ios examples build jobs to a dedicated script, which makes it easier to do non-trivial logic.