Use collapser around `root_tn` in `async_main()`

Replacing yet another loose-eg-flag. Also toss in a todo to maybe use
the unmasker around the `open_root_actor()` body.

tractor/_root.py

@@ -480,8 +480,11 @@ async def open_root_actor(
             async with (
                 collapse_eg(),
                 trio.open_nursery() as root_tn,
-            ):
 
+                # XXX, finally-footgun below?
+                # -> see note on why shielding.
+                # maybe_raise_from_masking_exc(),
+            ):
                 # `_runtime.async_main()` creates an internal nursery
                 # and blocks here until any underlying actor(-process)
                 # tree has terminated thereby conducting so called

tractor/_runtime.py

@@ -1484,9 +1484,8 @@ async def async_main(
 
             ipc_server: _server.IPCServer
             async with (
-                trio.open_nursery(
-                    strict_exception_groups=False,
-                ) as service_nursery,
+                collapse_eg(),
+                trio.open_nursery() as service_nursery,
                 _server.open_ipc_server(
                     parent_tn=service_nursery,
                     stream_handler_tn=service_nursery,