Fix node-gyp build: add PrivateTmp to lamassu-build service

modules/lamassu-lnbits.nix

@@ -270,6 +270,8 @@ in
         CI = "true";
         CC = "${pkgs.stdenv.cc}/bin/cc";
         CXX = "${pkgs.stdenv.cc}/bin/c++";
+        # Use content-addressable store to reduce disk usage
+        npm_config_cache = "${cfg.dataDir}/.npm-cache";
       };
 
       serviceConfig = {
@@ -285,6 +287,8 @@ in
         ProtectHome = true;
         NoNewPrivileges = true;
         ReadWritePaths = [ cfg.dataDir ];
+        # node-gyp needs writable /tmp for native module compilation
+        PrivateTmp = true;
       };
 
       script = ''