nixos/qemu-vm: set daemon user in register-nix-paths

The register-nix-paths service caused the nix database to be owned by
root, breaking unprivileged daemon support.

It is unclear why this worked previously.

nixos/modules/virtualisation/qemu-vm.nix

@@ -1307,6 +1307,8 @@ in
       serviceConfig = {
         Type = "oneshot";
         RemainAfterExit = true;
+        User = lib.mkIf (config.nix.daemonUser != "root") config.nix.daemonUser;
+        Group = lib.mkIf (config.nix.daemonGroup != "root") config.nix.daemonGroup;
       };
       script = ''
         if [[ "$(cat /proc/cmdline)" =~ regInfo=([^ ]*) ]]; then