truncate: process all files when one fails

[sylvestre]

Should make test truncate/multiple-files.sh pass

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/truncate/multiple-files is no longer failing!
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[cakebaker]

Congrats! The gnu test tests/truncate/multiple-files is no longer failing!

Kudos!