Motivation for this change

When I added support for socket activation to rspamd module in #34562 I missed that systemd deletes the RuntimeDirectory when the service is stopped. Since the default worker socket is in that directory it means that if the service is stopped the socket is deleted which means the socket unit needs to be restarted for the socket to be recreated.

Setting the RuntimeDirectoryPreserve option stops systemd from deleting the runtime directory so this change does that.

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.