fix(opencode): set LD_LIBRARY_PATH for prebuilt node bindings

The file watcher binding (and other node-precompiled .node modules
loaded via dlopen) failed with "libstdc++.so.6: cannot open shared
object file" because systemd services don't inherit the user shell's
LD path. Reuse the nix-ld library list so the service sees the same
common libraries unwrapped binaries get globally.

systems/x86_64-linux/amd/opencode.nix

@@ -35,6 +35,7 @@ in
 
     environment = {
       HOME = homeDir;
+      LD_LIBRARY_PATH = lib.makeLibraryPath config.programs.nix-ld.libraries;
     };
 
     serviceConfig = {