bugfix: switch archive timestamps to utc

we use utc for file timestamps and manifest timestamp,
so let's use utc for archives also.

src/borg/helpers/time.py

@@ -127,4 +127,4 @@ class OutputTimestamp:
 
 def archive_ts_now():
     """return tz-aware datetime obj for current time for usage as archive timestamp"""
-    return datetime.now().astimezone()  # local time / local timezone
+    return datetime.now(timezone.utc)  # utc time / utc timezone