Oof, fix walrus assign causes name-error edge case

Only warn log on a non-`trio` async lib when in the main thread to
avoid a name error when in the non-`asyncio` non-main-thread case.

=> To cherry into the `.pause_from_sync()` feature branch.

tractor/devx/_debug.py

@@ -190,11 +190,14 @@ class Lock:
         is_trio_main = (
             # TODO: since this is private, @oremanj says
             # we should just copy the impl for now..
-            trio._util.is_main_thread()
+            (is_main_thread := trio._util.is_main_thread())
             and
             (async_lib := sniffio.current_async_library()) == 'trio'
         )
-        if not is_trio_main:
+        if (
+            not is_trio_main
+            and is_main_thread
+        ):
             log.warning(
                 f'Current async-lib detected by `sniffio`: {async_lib}\n'
             )