Update `_runtime/_root` call-sigs to `nest_from_op()`

To match the new signature now already cherry-picked upstream.

The original commit was split after moving the original fn-params
changes into the upstream branch: `sclang_formatting`.

The orig commit was,

> ea53d03 Bah! just refine `devx.pformat.nest_from_op()`now

but was re-written to only contain the diff to calling code.

tractor/_root.py

@@ -521,7 +521,7 @@ async def open_root_actor(
 
                     op_nested_actor_repr: str = _pformat.nest_from_op(
                         input_op='>) ',
-                        tree_str=actor.pformat(),
+                        text=actor.pformat(),
                         nest_prefix='|_',
                     )
                     logger.info(

tractor/_runtime.py

@@ -1698,10 +1698,10 @@ async def async_main(
         )
 
     op_nested_actor_repr: str = _pformat.nest_from_op(
-        input_op=')> ',
-        tree_str=actor.pformat(),
+        input_op=')>',
+        text=actor.pformat(),
         nest_prefix='|_',
-        back_from_op=2,
+        nest_indent=1,  # under >
     )
     teardown_report += (
         'Actor runtime exited\n'