fix service.kresd fails due to kresd-cachedir hiatus

[tokudan]

Motivation for this change

Without this change the kresd-cachedir enters an endless loop of restarts until systemd completely fails it after several retries. This in turn may cause kresd to be stopped by systemd, potentially leaving the system without a dns resolver.

Only relevant for 17.09, because master uses different services.

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.

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