Fix deprecated utcnow() usage in build-many-glibcs.py

Running build-many-glibcs.py with Python 3.12 or later produces a
warning:

build-many-glibcs.py:566: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
  build_time = datetime.datetime.utcnow()

Replace with datetime.datetime.now(datetime.timezone.utc) (the
datetime.UTC constant is new in 3.11, so not suitable for use in this
script at present).