In the sandbox built for https://nixbuild.net, the coreutils build fails
because a failure in the df skip-rootfs test. The test failure is triggered by
the existance of a rootfs file system. However, I think that the test is faulty,
and I have reported it upstream in
https://lists.gnu.org/archive/html/bug-coreutils/2019-12/msg00000.html.

Disabling the test makes the coreutils build work in the nixbuild.net sandbox,
and I can't think of any negative impact disabling it can have. In normal nix
setups and in the normal nix sandbox, this test is not exercised anyway, since
there is no rootfs visible.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.