Use `macos-12` to build release wheels (#3264)
## Summary
GitHub has started to change `macos-latest` to `macos-14`. But
executables built on `macos-14` don't work on macOS 11 (see:
https://github.com/astral-sh/uv/issues/3261). This PR explicitly uses
`macos-12` instead (which is what we _intended_ to be using anyway).
Closes https://github.com/astral-sh/uv/issues/3261.
