sleep: Duration parsing does not support all float formats (e.g. hex)

Same as #7475, but for `sleep`. `sleep` isn't using the common duration parser, instead, it's using a parser from another crate (`fundu`)

GNU coreutils supports things like hex formatting:

```
sleep 0x1.1
sleep 0x1.1p-1
sleep 0x10.1
```

Not a huge deal, probably a pretty rare edge case that I doubt anybody cares about, but still worth sharing code with `timeout`.