PLT-5884 turn off darwin builds on ci

[ladamesny]

Pre-submit checklist:

• Branch
  • [ ] Tests are provided (if possible)
  • [ ] Commit sequence broadly makes sense
  • [ ] Key commits have useful messages
  • [ ] Relevant tickets are mentioned in commit messages
  • [ ] Formatting, materialized Nix files, PNG optimization, etc. are updated
• PR
  • [ ] Self-reviewed the diff
  • [ ] Useful pull request description
  • [ ] Reviewer requested