nixos/tests/systemd-repart: do not assume structured journal logs in
tests
systemd-repart and systemd-journal start up in parallel. This means
that quite often systemd-repart ends up logging to /dev/kmesg as opposed
to the journal.
This makes the tests very flakey (see e.g.
https://hydra.nixos.org/job/nixos/release-25.11/nixos.tests.systemd-repart.after-initrd.x86_64-linux)
Instead we switch to --syslog-identifier which is set for both
logs in /dev/kmesg and for journal logs