dhcp-identifier: always use a fixed machine-id while fuzzing

dhcp-identifier: always use a fixed machine-id while fuzzing

It's a follow-up to https://github.com/systemd/systemd/pull/10200 where
that fuzzer was introduced. At the time it was run regularly on machines
where machine-id wasn't present so it was kind of reproducible. Now
it's run on CIFuzz and CFLite using GHActions with the public OSS-Fuzz
corpora (based on that particular machine-id) so to fully utilize
those corpora it's necessary to use it always. Other than that
it makes it possible for fuzzers targeting outgoing packets
based on incoming packets like https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1795921
to get past client_parse_message on my machine :-)