Fix(executor): use timezone-aware object to represent datetime in UTC

[georgesittas]

This fixes a warning emitted on python ≥v3.12:

sqlglot/sqlglot/executor/env.py:243: 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).
  "UNIXTOTIME": null_if_any(lambda arg: datetime.datetime.utcfromtimestamp(arg))