sort: adapt error message to pass sort-files0-from

[cakebaker]

An expected error message in the GNU test sort-files0-from changed slightly in version 9.8. This PR adapts our error message to make the test pass again.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/sort/sort-files0-from is no longer failing!

[sylvestre]

i like these easy fix ;)

[codspeed-hq]

CodSpeed Performance Report

Merging #8748 will not alter performance

_{Comparing cakebaker:sort_fix_files0_from (7739598) with main (20c2779)¹}

Summary

✅ 93 untouched
⏩ 1 skipped²

Footnotes

1. No successful run was found on main (7739598) during the generation of this report, so 20c2779 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩