maintainers/scripts/update.nix: Print stderr on instantiation failure

For some reason `asyncio.create_subprocess_exec` freezes after failing previous call returns `CalledProcessError`.
Perhaps because `asyncio.gather` fails and cancels the later tasks, triggering https://github.com/python/cpython/issues/103847

maintainers/scripts/update.py

@@ -92,7 +92,15 @@ async def attr_instantiation_worker(
 ) -> tuple[Path, str]:
     async with semaphore:
         eprint(f"Instantiating {attr_path}…")
+        try:
             return (await nix_instantiate(attr_path), attr_path)
+        except Exception as e:
+            # Failure should normally terminate the script but
+            # looks like Python is buggy so we need to do it ourselves.
+            eprint(f"Failed to instantiate {attr_path}")
+            if e.stderr:
+                eprint(e.stderr.decode("utf-8"))
+            sys.exit(1)
 
 
 async def requisites_worker(