This change fixes the apt repository structure to avoid emiting warnings on
Ubuntu 18.04 (and potentially other versions). This requires a slight refactor
of the repository generation scripts, since we can no longer copy the same
release files for different "suites".
This should avoid the warning by setting the suite to the distribution:
https://github.com/Debian/apt/blob/master/apt-pkg/metaindex.cc#L75
This change also moves over to the standardized Makefile entrypoint, which
makes settings clearer and enables local testing.
PiperOrigin-RevId: 313817017
