nixos/public-inbox: make coderepo paths accessible

nixos/modules/services/mail/public-inbox.nix

···
           after = [ "public-inbox-init.service" "public-inbox-watch.service" ];
           requires = [ "public-inbox-init.service" ];
           serviceConfig = {
+            BindPathsReadOnly =
+              map (c: c.dir) (lib.attrValues cfg.settings.coderepo);
             ExecStart = escapeShellArgs (
               [ "${cfg.package}/bin/public-inbox-httpd" ] ++
               cfg.http.args ++