chore(ci): don't run update contributors on forked PRs

[erezrokah]

🎉 Thanks for submitting a pull request! 🎉

Summary

The update contributors action doesn't work well on forked PRs for 2 reasons:

1. with-ref doesn't seem to work (haven't looked into a solution). See https://github.com/netlify/cli/runs/5230803617?check_suite_focus=true#step:2:48
2. Using the built-in GitHub token doesn't trigger new workflow runs. We can't generate our custom token since GitHub secrets are not exposed to forked PRs.

This PR disables the action, resulting in the package.json being updated in the next release PR. See #4322

---

For us to review and ship your PR efficiently, please perform the following steps:

• Open a bug/issue before writing your code 🧑‍💻. This ensures we can discuss the changes and get feedback from everyone that should be involved. If you`re fixing a typo or something that`s on fire 🔥 (e.g. incident related), you can skip this step.
• Read the contribution guidelines 📖. This ensures your code follows our style guide and
  passes our tests.
• Update or add tests (if any source code was changed or added) 🧪
• Update or add documentation (if features were changed or added) 📝
• Make sure the status checks below are successful ✅

A picture of a cute animal (not mandatory, but encouraged)
🐶

[github-actions]

📊 Benchmark results

Comparing with c7b28c5

Package size: 442 MB

⬇️ 0.00% decrease vs. c7b28c5

^                                  510 MB  510 MB  510 MB  509 MB                                         
│                                   ┌──┐    ┌──┐    ┌──┐    ┌──┐                                          
│                                   |  |    |  |    |  |    |  |                                          
│ ─442 MB──442 MB──442 MB──442 MB───┼──┼────┼──┼────┼──┼────┼──┼───439 MB──439 MB──439 MB──439 MB──442 MB─
│   ┌──┐    ┌──┐    ┌──┐    ┌──┐    |  |    |  |    |  |    |  |    ┌──┐    ┌──┐    ┌──┐    ┌──┐    ┌──┐  
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└───┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴──>
    T-12    T-11    T-10    T-9     T-8     T-7     T-6     T-5     T-4     T-3     T-2     T-1      T    

Legend

• T-30 (c7b28c5): 442 MB
• T-29 (03277be): 442 MB
• T-28 (cbb1e43): 442 MB
• T-27 (b6fefc3): 442 MB
• T-26 (89194de): 442 MB
• T-25 (9735540): 446 MB
• T-24 (7036285): 446 MB
• T-23 (78ca40b): 442 MB
• T-22 (ac55ed3): 442 MB
• T-21 (f8e1953): 442 MB
• T-20 (8b697d9): 442 MB
• T-19 (62d4e01): 442 MB
• T-18 (4e1ca42): 442 MB
• T-17 (6212d30): 442 MB
• T-16 (d961855): 442 MB
• T-15 (fa3980d): 442 MB
• T-14 (9754f66): 442 MB
• T-13 (2913759): 442 MB
• T-12 (050bf90): 442 MB
• T-11 (b74c0ee): 442 MB
• T-10 (d1577f6): 442 MB
• T-9 (1fb1f34): 442 MB
• T-8 (05a0e92): 510 MB
• T-7 (bd52b3e): 510 MB
• T-6 (96446a9): 510 MB
• T-5 (f5bd8f2): 509 MB
• T-4 (3279dc3): 439 MB
• T-3 (0b2d2c9): 439 MB
• T-2 (9773787): 439 MB
• T-1 (7b6b94a): 439 MB
• T (current commit): 442 MB