gitea: git protocol v2 over ssh

cf. https://docs.gitea.io/en-us/config-cheat-sheet/#git-git, entry for
option ENABLE_AUTO_GIT_WIRE_PROTOCOL

services/gitea.nix

@@ -111,6 +111,9 @@
           addr = "192.168.100.10";
           port = 22;
         } ];
+        extraConfig = ''
+          AcceptEnv GIT_PROTOCOL
+        '';
       };
     })).config.system.build.toplevel;
   };