Use the macos-12 runner.

[brnnmrls]

Update spm-build-test runner to macos-12.

macos-11 was deprecated according to: https://github.blog/changelog/2024-05-20-actions-upcoming-changes-to-github-hosted-macos-runners/#macos-11-deprecation-and-removal

Updating this to macos-12 shows our codecov step is failing but found a workaround and explanation here: https://github.com/codecov/codecov-action/issues/1549