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)