How do I put assign supplementary groups to nginx user?

[Jayesh Bhoot]

Hello,

I am setting up a git server with Guix System with the following configuration:

- A git user with home directory set to /srv/git, so that git repos can be 
hosted from /srv, and the repo urls can have the shortest path possible, like 
git@server:test-repo.git.
- A git group to which the git user is assigned.
- cgit-service-type to serve a read-only view of the repos, with nginx acting 
as the server.

In order to serve the repos, nginx needs access to /srv/git. But, /srv/git, 
being a home directory, has the configuration of 700 git:git by default. I need 
to loosen up its permissions to at least 750 so that the git group members can 
read the directory, and add nginx user to the git group.

How do I encode the following withing the system-configuration.scm?

- add nginx user to git supplementary group. Neither (cgit-service-type) not 
(nginx-configuration) provide option to edit nginx's supplementary group, and 
%nginx-accounts does not seem to be exported.
- modify permissions of home directory /srv/git to 750. (user-account) does not 
seem to have this option.

I saw a similar question in the mailing list from 2017, but that one didn't end 
with a solution: 
https://lists.gnu.org/archive/html/help-guix/2017-06/msg00052.html

P.S.: Please interpert all of the above with the context that I have spent only 
about a weekend with Guix and Guile.