`systemd-run` fails with unknown `--message` option

[p5]

It seems --message is not a valid argument to systemd-run.
The closest seems to be --description, so I think we should use that instead.

❯ systemctl status bootc-fetch-apply-updates.service
× bootc-fetch-apply-updates.service - Apply bootc updates
     Loaded: loaded (/usr/lib/systemd/system/bootc-fetch-apply-updates.service; static)
    Drop-In: /usr/lib/systemd/system/service.d
             └─10-timeout-abort.conf
     Active: failed (Result: exit-code) since Thu 2025-07-17 10:59:12 BST; 1s ago
 Invocation: 1e761d44e3584448a4f5e5bacbe157af
       Docs: man:bootc(8)
    Process: 33040 ExecStart=/usr/bin/bootc upgrade --apply --quiet (code=exited, status=1/FAILURE)
   Main PID: 33040 (code=exited, status=1/FAILURE)
   Mem peak: 34.8M
        CPU: 169ms

Jul 17 10:59:11 UNKNOWN systemd[1]: Starting bootc-fetch-apply-updates.service - Apply bootc updates...
Jul 17 10:59:11 UNKNOWN bootc[33040]: Fetching ostree-image-signed:docker://ghcr.io/rsturla/eternal-linux/lumina:42-nvidia
Jul 17 10:59:12 UNKNOWN bootc[33040]: No changes in ostree-image-signed:docker://ghcr.io/rsturla/eternal-linux/lumina:42-nvidia => sha256:327f86b2de9e245526104864ff4b276c67d290a5c7479eb1ee5059fbbc50ce98
Jul 17 10:59:12 UNKNOWN bootc[33040]: Staged update present, not changed.
Jul 17 10:59:12 UNKNOWN bootc[33040]: Rebooting system
Jul 17 10:59:12 UNKNOWN bootc[33083]: systemd-run: unrecognized option '--message=Initiated by bootc'
Jul 17 10:59:12 UNKNOWN bootc[33040]: error: Upgrading: Initiating reboot: Task Rebooting system failed: ExitStatus(unix_wait_status(256))
Jul 17 10:59:12 UNKNOWN systemd[1]: bootc-fetch-apply-updates.service: Main process exited, code=exited, status=1/FAILURE
Jul 17 10:59:12 UNKNOWN systemd[1]: bootc-fetch-apply-updates.service: Failed with result 'exit-code'.
Jul 17 10:59:12 UNKNOWN systemd[1]: Failed to start bootc-fetch-apply-updates.service - Apply bootc updates.