coreutils: fix SIGHUP in certain flaky tests

[mdaniels5757]

Context: https://github.com/NixOS/nixpkgs/pull/487081#issuecomment-3868051076 and ensuing discussion
Context: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=80477

Fixes: #495290

Cc: @chrisheib

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[uninsane]

fixes coreutils build, including coreutils.override { singleBinary = true; }, coreutils.override { singleBinary = "shebangs"; }