fix(ds): don't use env var for data dir default value

Fixes https://github.com/emqx/emqx/pull/12380

apps/emqx/src/emqx_persistent_message.erl

@@ -61,13 +61,13 @@ force_ds() ->
     emqx_config:get([session_persistence, force_persistence]).
 
 storage_backend(#{
-    builtin := #{
+    builtin := Opts = #{
         enable := true,
-        data_dir := DataDir,
         n_shards := NShards,
         replication_factor := ReplicationFactor
     }
 }) ->
+    DataDir = maps:get(data_dir, Opts, emqx:data_dir()),
     #{
         backend => builtin,
         data_dir => DataDir,

apps/emqx/src/emqx_schema.erl

@@ -1896,11 +1896,8 @@ fields("session_storage_backend_builtin") ->
                 string(),
                 #{
                     desc => ?DESC(session_builtin_data_dir),
-                    default => <<"${EMQX_DATA_DIR}">>,
-                    importance => ?IMPORTANCE_LOW,
-                    converter => fun(Path, Opts) ->
-                        naive_env_interpolation(ensure_unicode_path(Path, Opts))
-                    end
+                    required => false,
+                    importance => ?IMPORTANCE_LOW
                 }
             )},
         {"n_shards",