Revert to using macOS 12 for VR tests

[rmccar]

What is the context of this PR?

Fixes: #3159

Github seems to have recently updated their macos-latest runner to use macOS 14 (as explained here April 24 - https://github.blog/changelog/2024-01-30-github-actions-macos-14-sonoma-is-now-available/). These runners use the new Apple silicon CPUs which are currently incompatible with running colima (https://github.com/abiosoft/colima/issues/970) which is what we use to run our VR tests.

This PR specifies that we use macOS 12 rather than "Latest".

How to review this PR

• Check the VR tests on another PR see the colima error
• See that the VR tests run and pass on this PR

Checklist

This needs to be completed by the person raising the PR.

• [x] I have selected the correct Assignee
• [x] I have linked the correct Issue

[netlify[bot]]

✅ Deploy Preview for ons-design-system-preview ready!

Name	Link
🔨 Latest commit	e54975f364d7ab8168edb7d885c58020e72267f7
🔍 Latest deploy log	https://app.netlify.com/sites/ons-design-system-preview/deploys/662a599d8fb7e300081922dd
😎 Deploy Preview	https://deploy-preview-3157--ons-design-system-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.