GitHub made arm64 runners generally available and changed macos-latest label to point to them.
The runner architecture is coupled with GH-hosted runners OS version: macos-13 is the last one to run on x64.

This PR essentially brings back the previous behavior, by explicitly requesting that all our x64 macOS jobs are run on macos-12 (as was before). 
We should eventually migrate to macos-13 for x64 macOS and macos-14/macos-latest for arm64 macOS. However, this leads to issues with `npm install` getting stuck, so it should be probably reattempted after the CI rework.