bug#74681: seatd-service-type: error with elogind-service-type

[fourfivemg]

Adding both to the `services` section of your `(operating-system)` form, like
this:

```
  (services
    (append (list
             (service elogind-service-type)
             (service seatd-service-type))
            %base-services))
```

...will produce this error on running `sudo guix system reconfigure
/etc/config.scm`:

```
guix system: error: service 'file-system-/sys/fs/cgroup' provided more than once
```

...and the command will fail with no further error messages and exit code 1.

Not sure how to figure this out further. Tried `guix system build -K`, `--debug`
and `--verbosity` options; no change in output.

A different config producing the same error message was reported here:
https://www.reddit.com/r/GUIX/comments/14fy6ac/configuration_file_returns_error_when_trying_to/