touch: Fix leap second year overflow

[mtvb]

Replaced the panicking dt += 1.second() with dt.checked_add(1.second()), mapping the overflow case to the same USimpleError/invalid date format error every other unparseable -t value already produces.

Fixes #14212

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.