libutil: Fix error message (backport #12758)

[mergify]

I encountered this with a misconfigured libutil. I doubt that a non-lutimes config is viable, because tests were failing.

EDIT: I get the misconfiguration reproducibly with .#native-clangStdenv; need to look into why it happens
EDIT: nix build .#nix-everything-clangStdenv passes with this PR

Motivation

Context

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This is an automatic backport of pull request #12758 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 1cffcd9 has failed:

On branch mergify/bp/2.24-maintenance/pr-12758
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 1cffcd91a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libutil/file-system.cc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[roberth]

Unimportant for backport