du: Parse "--time-style" only when necessary

Discovered while working on ls in #6162. Example:

```console
$ du -s --time-style=banana
26703160	.
$ cargo run du -s --time-style=banana
du: invalid argument 'banana' for 'time style'
Valid arguments are:
- 'full-iso'
- 'long-iso'
- 'iso'
Try 'target/debug/coreutils du --help' for more information.
[$? = 1]
```