services.xray: pass the settings file with systemd loadCredential

It enables passing a sops-nix secret as a `settingsFile`
@see https://github.com/Mic92/sops-nix/issues/198.

By default sops-nix secrets are accessible by only root. We can change owner to another user, but the xray service is defined with `dynamicUser=true`, which means, there is no user in the compile time.

Systemd `loadCredential` passes the secret file to the service, which is exactly what we need here.

nixos/modules/services/networking/xray.nix

@@ -80,9 +80,12 @@ with lib;
       description = "xray Daemon";
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
+      script = ''
+        exec "${cfg.package}/bin/xray" -config "$CREDENTIALS_DIRECTORY/config.json"
+      '';
       serviceConfig = {
         DynamicUser = true;
-        ExecStart = "${cfg.package}/bin/xray -config ${settingsFile}";
+        LoadCredential = "config.json:${settingsFile}";
         CapabilityBoundingSet = "CAP_NET_ADMIN CAP_NET_BIND_SERVICE";
         AmbientCapabilities = "CAP_NET_ADMIN CAP_NET_BIND_SERVICE";
         NoNewPrivileges = true;