Motivation for this change

To fix errors such as

Could not delete unused user data directory /var/lib/lightdm-data/.cache: Failed to execute child process “/bin/rm” (No such file or directory)
Could not delete unused user data directory /var/lib/lightdm-data/.Xauthority: Failed to execute child process “/bin/rm” (No such file or directory)
Could not delete unused user data directory /var/lib/lightdm-data/lightdm: Failed to execute child process “/bin/rm” (No such file or directory)

that you can see in journalctl -u display-manager.service. I believe this should be backported to 17.09 (where I successfully run this code) as the bug also manifests there.

Maintainer CC: @ocharles @wkennington

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.