`datafusion.execution.time_zone` is not used for basic time zone inference

### Describe the bug

I would expect that setting `SET datafusion.execution.time_zone = '+08:00'` would lead to `SELECT '2024-11-01T00:00:00.0+00:00'::timestamptz - '2024-11-01T00:00:00.0'::timestamp` to give `8 hours` (that is, the same as `SELECT '2024-11-01T00:00:00.0+00:00'::timestamptz - '2024-11-01T00:00:00.0+08:00'::timestamptz`). Some things do work e.g. `SELECT '2024-11-01T00:00:00.0'::timestamptz` does give `2024-11-01T00:00:00+08:00`.

### To Reproduce

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### Expected behavior

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### Additional context

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