nixos/gogs: allow git operations over ssh

Without `ROOT_PATH` set, `gogs serv` tries to open logs in writing in
its store directory. This blocks cloning or pushing over ssh, and
results in a gogs internal error.

(cherry picked from commit b59570eac05b65e23b6a0ccc8a665027d740b1d9)

nixos/modules/services/misc/gogs.nix

@@ -35,6 +35,9 @@ let
     SECRET_KEY = #secretkey#
     INSTALL_LOCK = true
 
+    [log]
+    ROOT_PATH = ${cfg.stateDir}/log
+
     ${cfg.extraConfig}
   '';
 in