Fix usage of deprecated get_child_watcher

[nanonyme]

There's a new warning in BuildStream output

  /usr/lib64/python3.12/asyncio/events.py:828: DeprecationWarning: 'get_child_watcher' is deprecated as of Python 3.12 and will be removed in Python 3.14.

[nanonyme]

This is used in

buildstream/src/buildstream/_scheduler/scheduler.py

Line 163 in 936124f

_watcher = asyncio.get_child_watcher()

[nanonyme]

Looks like python/cpython#94597 is the issue with full context.