switch to `Etc/Unknown` when `TimeZone::system()` could not find a system configured time zone

[BurntSushi]

This time zone is the same as UTC, but it's now officially reserved by the IANA Time Zone Database such that it will be a valid identifier. I believe icu uses it as a useful indicator that a system time zone could not be found instead of just silently using UTC.

[BurntSushi]

I also wonder also if Etc/Unknown should result in a -00:00 offset (where today UTC results in +00:00). This might be an issue in practice since a jiff::tz::Offset cannot differentiate between negative and positive zero.