Make embedded timestamps timezone independent

[kpcyrd]

This is a follow-up for #61, the value of TZ still has an impact on the binary (removing the env::set_var call in one of the tests makes the test fail as well):

---- time::tests::test_timezone_utc stdout ----
thread 'time::tests::test_timezone_utc' panicked at 'assertion failed: `(left == right)`
  left: `"2021-08-04 14:34:03 +02:00"`,
 right: `"2021-08-04 12:34:03 +00:00"`', src/time.rs:46:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

With this patch SOURCE_DATE_EPOCH makes the embedded timestamp use UTC without build-time localization of the timezone. It also removes build-time localization when embedding the most-recent-git-commit timestamp (this didn't show up in our tests because we've tested with a sourcecode from a tarball).

The code could be simplified by always using UTC, then localize the timezone at runtime if needed.

I've dropped one of the tests because calling Local::now(); after now_data_time(); is always going to give you a greater value, unless SOURCE_DATE_EPOCH is explicitly set to a value in the future.

Thanks!