Fix Launch.runtime_secs TypeError with mixed tz-aware/naive datetimes

[janosh]

Normalize timezone-naive datetimes to UTC-aware in Launch._get_time() to prevent
TypeError when subtracting mixed naive/aware datetimes from MongoDB.

Traceback (most recent call last):
  File "/Users/matt/git/periodic-mono/.venv/bin/lpad", line 10, in <module>
    sys.exit(lpad())
             ^^^^^^
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/scripts/lpad_run.py", line 1578, in lpad
    args.func(args)
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/scripts/lpad_run.py", line 503, in detect_lostruns
    fl, ff, fi = lp.detect_lostruns(
                 ^^^^^^^^^^^^^^^^^^^
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/core/launchpad.py", line 1401, in detect_lostruns
    self.mark_fizzled(lid)
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/core/launchpad.py", line 1290, in mark_fizzled
    self.complete_launch(launch_id, state="FIZZLED")
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/core/launchpad.py", line 1572, in complete_launch
    self.launches.find_one_and_replace({"launch_id": m_launch.launch_id}, m_launch.to_db_dict(), upsert=True)
                                                                          ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/utilities/fw_serializers.py", line 153, in _decorator
    m_dict = func(self, *args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/core/firework.py", line 610, in to_db_dict
    m_d["runtime_secs"] = self.runtime_secs
                          ^^^^^^^^^^^^^^^^^
  File "/Users/matt/git/periodic-mono/.venv/lib/python3.12/site-packages/fireworks/core/firework.py", line 576, in runtime_secs
    return (end - start).total_seconds()
            ~~~~^~~~~~~
TypeError: can't subtract offset-naive and offset-aware datetimes