Create a file we can check at runtime, and identify if the
environment we run against is one of our CI build images.
Useful mostly for our internal scritps so we can match against
metadata rather than heuristics, ex. if /subprojects exists
Conceptually similar to /.flatpak-info
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8763>
