date: fix handling of case-change flags in locale format specifiers

[CrazyRoka]

Previously, applying flags like '^' (uppercase) or '#' (swap case) to locale-dependent specifiers (like %c, %x, %X) failed to affect the output. This occurred because the implementation passed these specifiers directly to the underlying formatter, which treats %c as a single unit without inheriting flags for its expanded components.

For example, LC_ALL=C TZ=UTC cargo run --bin coreutils date -d 2024-01-01 +%^c produced "2024 M01 1, Mon 00:00:00", whereas GNU date produces "MON JAN 1 00:00:00 2024".

This change pre-processes the format string to expand %c, %x, and %X using nl_langinfo.

This aligns behavior with GNU coreutils and we are now passing subfmt-up1 edge case in tests/date/date.pl

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[sylvestre]

lot of jobs are failing

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/id/smack was skipped on 'main' but is now failing.
Note: The gnu test tests/mkdir/smack-no-root was skipped on 'main' but is now failing.
Note: The gnu test tests/mkdir/smack-root was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tty/tty-eof. tests/tty/tty-eof is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)