Here's the code I have for /etc/os-release. I defined %os-release-file
at the top to keep my actual services list a bit shorter. The '@'
notation is a short-cut to not need to actually import (guix packages)
or (gnu packages package-management) in my config.

Then later when adding the other services the 'simple-service' can just
be added in and it'll be combined with the other 'etc-service-type'
services to create a unified /etc.

(define %os-release-file
  (plain-file "os-release"
              (string-append
                "NAME=\"Guix System\"\n"
                "PRETTY_NAME=\"Guix System\"\n"
                "VERSION=\""((@ (guix packages) package-version) (@ (gnu packages package-management) guix))"\"\n"
                "ID=guix\n"
                "HOME_URL=\"https://www.gnu.org/software/guix/\"\n"
                "SUPPORT_URL=\"https://www.gnu.org/software/guix/help/\"\n"
                "BUG_REPORT_URL=\"mailto:bug-guix@gnu.org\"\n")))

(simple-service 'os-release etc-service-type
                `(("os-release" ,%os-release-file)))

and here's what it looks like:

efraim@E5400 ~$ cat /etc/os-release 
NAME="Guix System"
PRETTY_NAME="Guix System"
VERSION="1.0.1-6.0ed97e6"
ID=guix
HOME_URL="https://www.gnu.org/software/guix/"
SUPPORT_URL="https://www.gnu.org/software/guix/help/"
BUG_REPORT_URL="mailto:bug-guix@gnu.org"

-- 
Efraim Flashner   <efraim@flashner.co.il>   אפרים פלשנר
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