Ensure output is compatible with systemd-sysupdate

Ensure output is compatible with systemd-sysupdate

When using `systemd-sysupdate` it is wise to use split artifacts and to
compress artifacts. These changes ensure that split artifacts are compressed
and the file paths are stored in the checksum.

Please note that `systemd-sysupdate` expects a checksum name of `SHA256SUMS`
and uses the contained file names to determine which files to download. The
compression suffix is also used to determine how to decompress files.

Additional CI checks have been added to make sure no regressions in output
format occur.