Pin the mac runners to 12

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

macos-latest used to refer to macos-12, but has since changed to macos-14-arm64. It'll take some effort to upgrade the logic for that jump, so for now pin back to 12.

---

Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/8824930809

---

Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

---