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).

scripts/build-many-glibcs.py

@@ -563,7 +563,7 @@ class Context(object):
                 print(config.name, config.compiler.name)
             return
         self.clear_last_build_state(action)
-        build_time = datetime.datetime.utcnow()
+        build_time = datetime.datetime.now(datetime.timezone.utc)
         if action == 'host-libraries':
             build_components = ('gmp', 'mpfr', 'mpc')
             old_components = ()