There are a lot of challenges around timezone handling, and IMHO some refactoring will be needed.

Decided to focus on #7504 for now: we use a system timezone database is available through tzfile.

Also, added some tests, and a bunch of TODO in the code to give pointers to the next person fixing this (maybe me...).

Shrinks date/ls by about 2 megabytes on Linux:

cargo build -r -p uu_date && cargo build -r -p uu_ls && ls -l target/release/{date,ls}
-rwxr-xr-x 2 drinkcat drinkcat 4828144 Apr 27 13:57 target/release/date
-rwxr-xr-x 2 drinkcat drinkcat 3769336 Apr 27 13:57 target/release/ls
## after
-rwxr-xr-x 2 drinkcat drinkcat 2949080 Apr 27 13:58 target/release/date
-rwxr-xr-x 2 drinkcat drinkcat 1894176 Apr 27 13:59 target/release/ls

uucore: custom_tz_fmt: test: Setting TZ is not reliable

Not 100% clear why, but setting TZ in CI is not fully reliable,
so let's pass a parameter instead.

Cross.toml: Install tzdata in container

Linux tests require that now, as we now assume /usr/share/zoneinfo
is present.

uucore: custom_tz_fmt: Set an embed_tz feature based on platform

Instead of repeating complex logic (and its inverse), set a
feature in build.rs.

fuzz/Cargo.lock: Update

uucore: Use tzfile instead of chrono-tz

Unless we're on Windows.

Fixes #7504, reduces ls/date executable size by about 2 MB.

uucore: custom_tz_fmt: use format("%Z")

uucore: custom_tz_fmt: Add abbreviation tests

This function will still need some love, but at least with this,
we have basic tests.

uucore: custom_tz_fmt: Improve TZ environment variable handling

iana_time_zone::get_timezone doesn't cover all cases, so we need
to handle some of the timezone parsing manually.

Also add a bunch of TODO, this function is by no means complete.

(change similar to #7597 for this file)