sort: fix panic when collator not available in worker threads

[sylvestre]

Fixes: #10909

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/factor/t10. tests/factor/t10 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)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/pr/bounded-memory is no longer failing!
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!

[ChrisDryden]

I think this is a good change to make for being more resilient, but I think the actual error is in the:

.is_some_and(|enc| enc.to_lowercase() == "utf-8")

Where we should probably modify it to also accept without the hyphen:

  .is_some_and(|enc| {
      let lower = enc.to_lowercase();
      lower == "utf-8" || lower == "utf8"
  })