date: fix -s UTC conversion losing timezone offset

[yachi]

Fixes #10800

-s with a timezone offset used datetime().to_zoned(UTC) which strips the offset and reinterprets wall-clock time as UTC. Changed to timestamp().to_zoned(UTC) to preserve the absolute instant.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/usage_vs_getopt (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)

[ChrisDryden]

I don't think this actually tests anything, it just checks to see if the user is root and fails. Maybe it would be better to add a unit test for this for the conversion?

[ChrisDryden]

I see there's other integration tests in here that use the -s flag, can you follow the format of those tests?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/mv/mv-special-1. tests/mv/mv-special-1 is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/pr/bounded-memory is no longer failing!
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/pipe-f. tests/tail/pipe-f is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/factor/t33 is no longer failing!
Congrats! The gnu test tests/factor/t34 is no longer failing!
Congrats! The gnu test tests/factor/t36 is no longer failing!
Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!