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
diff --git a/tools/releasetools/build_image.py b/tools/releasetools/build_image.py
index b6c96c4..08b4d6a 100755
--- a/tools/releasetools/build_image.py
+++ b/tools/releasetools/build_image.py
@@ -49,8 +49,8 @@
 # Use a fixed timestamp (01/01/2009 00:00:00 UTC) for files when packaging
 # images. (b/24377993, b/80600931)
 FIXED_FILE_TIMESTAMP = int((
-    datetime.datetime(2009, 1, 1, 0, 0, 0, 0, None) -
-    datetime.datetime.utcfromtimestamp(0)).total_seconds())
+    datetime.datetime(2009, 1, 1, 0, 0, 0, 0, datetime.UTC) -
+    datetime.datetime.fromtimestamp(0, datetime.UTC)).total_seconds())
 
 
 class BuildImageError(Exception):