bug#68939: sddm does not start after update to 0.20.0

[Jonathan Brielmaier]

Today I rebooted my workstation running Guix System and somehow sddm
didn't started anymore.

I quickly found a suspicious commit:
05fad99a431c6708ec18c7d0db67178bd8d36597
gnu: sddm: Update to 0.20.0.

Then I pulled and reconfigured to this commit (Generation 14) and to one
commit before (Generation 13). And indeed the sddm update to 0.20.0
breaks my system.

```
Generation 13   Feb 05 2024 23:00:35
  file name: /var/guix/profiles/system-13-link
  canonical file name: /gnu/store/hq4bd4vzlyvnwblc5w2jpll4xn7nmzrd-system
  label: GNU with Linux 6.6.8
  bootloader: grub-efi
  root device: /dev/mapper/cryptroot
  kernel: /gnu/store/nn8dir9d1d025r123ib11jfsxxc714fl-linux-6.6.8/bzImage
  channels:
    guix:
      repository URL: https://git.savannah.gnu.org/git/guix.git
      commit: f96590fb3662300058022f0aaee594e766f8941f
  configuration file:
/gnu/store/4n81nr9bi0jn5vqfpa3s4y4wzci1zxkf-configuration.scm

Generation 14   Feb 05 2024 23:07:31
  file name: /var/guix/profiles/system-14-link
  canonical file name: /gnu/store/ks1f2wg4gb4sn2gv1ag0f3spp5xjv5gq-system
[ part is identical to Gen 13 ]
  channels:
    guix:
      repository URL: https://git.savannah.gnu.org/git/guix.git
      commit: 05fad99a431c6708ec18c7d0db67178bd8d36597
  configuration file:
/gnu/store/4n81nr9bi0jn5vqfpa3s4y4wzci1zxkf-configuration.scm
```

The relevant part of my configuration.scm:
```
(service sddm-service-type
  (sddm-configuration
    (display-server "wayland")))
```

There is an upstream bug report which is maybe relevant:
https://github.com/sddm/sddm/issues/1456

@Feng Shu: I CC'ed you for this bug as you updated the sddm package.
Maybe you are not affected by this issue or have a workaround :)

~Jonathan