open-stage-control: pin to nodejs_18 for now

[lilyinstarlight]

Description of changes

The C++ plugin only supports Node.js v18 for now. Fixes currently failing build in Hydra: https://hydra.nixos.org/build/243768070

Note that it still has some issues with Electron 26+ due to electron/remote#153, but it's been like that for a bit now and fixing that will require upstream to bump their deps. It is still just as usable as it was before the nodejs 18 -> 20 bump, since setting options on CLI bypasses the issue as before

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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