nuc: Fix git server ownership and permissions

Define the `git` as system user and delete its home directory.  Make it
part of the group `git`.

Make the git repositories owned by git user and group and adjust the
permissions:

    chown -R git:git /srv/git
    find /srv/git -type d -exec chmod 750 {} \;
    find /srv/git -type f -exec chmod 640 {} \;

Finally, run `cgit` as the group `git` so it can read the repositories.

modules/git.nix

@@ -7,16 +7,18 @@ in
   imports = [ ./consul-catalog.nix ];
 
   users.users.git = {
-    isNormalUser = true;
+    isSystemUser = true;
+    group = "git";
     shell = "${pkgs.git}/bin/git-shell";
     openssh.authorizedKeys.keys = (import ./keys.nix).dwagner;
   };
+  users.groups.git = { };
 
   services = {
     cgit.git = {
       enable = true;
       user = "cgit";
-      group = "cgit";
+      group = "git";
       scanPath = "/srv/git";
       settings = {
         enable-git-config = true;