Use same datetime object for {now} and {utcnow} (fixes #3548)

{now} and {utcnow} should point to the same exact momemt, but they don't
because .now() and .utcnow() create two different objects at different
times. Although the difference will be on the order of microseconds on all
but the slowest machines, this bug still tickles my inner pedant...

src/borg/helpers.py

@@ -665,14 +665,14 @@ def format_line(format, data):
 def replace_placeholders(text):
     """Replace placeholders in text with their values."""
     from .platform import fqdn
-    current_time = datetime.now()
+    current_time = datetime.now(timezone.utc)
     data = {
         'pid': os.getpid(),
         'fqdn': fqdn,
         'reverse-fqdn': '.'.join(reversed(fqdn.split('.'))),
         'hostname': socket.gethostname(),
-        'now': DatetimeWrapper(current_time.now()),
-        'utcnow': DatetimeWrapper(current_time.utcnow()),
+        'now': DatetimeWrapper(current_time.astimezone(None)),
+        'utcnow': DatetimeWrapper(current_time),
         'user': uid2user(os.getuid(), os.getuid()),
         'uuid4': str(uuid.uuid4()),
         'borgversion': borg_version,