c3d2-web: use lib.mkOptionDefault instead of options.*.default

hosts/c3d2-web/default.nix

@@ -1,4 +1,4 @@
-{ config, hostRegistry, options, pkgs, ... }:
+{ config, hostRegistry, lib, options, pkgs, ... }:
 let
   webroot = "/var/www";
 in
@@ -9,7 +9,7 @@ in
   };
   c3d2.deployment = {
     # /tmp is to small for drone to clone the repo even with depth
-    mounts = options.c3d2.deployment.mounts.default ++ [ "tmp" ];
+    mounts = lib.mkOptionDefault ++ [ "tmp" ];
     server = "server10";
   };