Fix deprecated utcfromtimestamp() usage.

Fixes the following build warning:

build_image.py:53: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).

Bug: 402676308
Test: presubmit
Change-Id: I5fe76402231a43aedf5ff5b3ad945fc117d64ffa