Hello Guix!

I tried to add (service colord-service) to my config and I got a
error back :-(. I reduced my system to the minimum but the problem is
persist.

Here's the backtrace I get with the colord-system.scm config attached:

```
$ guix system build colord-system.scm
(snip)
27692 entries processed in 10.3 s
successfully built /gnu/store/76v9hszcal0llfs7q8yapp6v0rnjwmkr-manual-database.drv
building /gnu/store/xs3khl253kzrf14cn2jz5hpabd69j88v-dbus-system-services.drv...
find-files: /gnu/store/nqr4xi6yyxzlyxmpvq668ckiksf0k96a-wpa-supplicant-2.9/share/dbus-1/: No such file or directory
find-files: /gnu/store/hpgsfyn3qn0lxp1p9nwcg6fbvslwpf59-gdm-3.30.3/share/dbus-1/: No such file or directory
Backtrace:
           3 (primitive-load "/gnu/store/r5qblihnlj5ysy47k94a6s316mf?")
In ice-9/eval.scm:
    619:8  2 (_ #f)
In srfi/srfi-1.scm:
    640:9  1 (for-each #<procedure 7ffff3cb97e0 at ice-9/eval.scm:3?> ?)
In unknown file:
           0 (symlink "/gnu/store/9641z2pcm85gx0xqi51i78m6k229nryr-?" ?)

ERROR: In procedure symlink:
In procedure symlink: File exists
builder for `/gnu/store/xs3khl253kzrf14cn2jz5hpabd69j88v-dbus-system-services.drv' failed with exit code 1
build of /gnu/store/xs3khl253kzrf14cn2jz5hpabd69j88v-dbus-system-services.drv failed
View build log at '/var/log/guix/drvs/xs/3khl253kzrf14cn2jz5hpabd69j88v-dbus-system-services.drv.bz2'.
cannot build derivation `/gnu/store/sla70asv0sf2c7gn4a5v50qkmy019703-dbus-configuration.drv': 1 dependencies couldn't be built
building /gnu/store/wq1naiygmpa01hn201l2i6ihd1bz44l1-etc-polkit-1.drv...
cannot build derivation `/gnu/store/vmnby0bcy575pylvbw5lva4k7zyrxrgd-etc.drv': 1 dependencies couldn't be built
building /gnu/store/4zr21fl9wsyy591rglgw47yzw9nnia2g-gdm-custom.conf.drv...
cannot build derivation `/gnu/store/yc1mdlzqqwd24x7gkxa9n6qfmjzx83zg-system.drv': 1 dependencies couldn't be built
guix system: error: build of `/gnu/store/yc1mdlzqqwd24x7gkxa9n6qfmjzx83zg-system.drv' failed
```

Do you know what's going on?

Thanks!
Pierre