.github(tests): bump macos-12 to macos-13

[vaeng]

GitHub dropped macOS-12 support. Workflow needs updating to 14.

See: https://forum.exercism.org/t/github-actions-runner-breaking-changes-announced/12612

[keiravillekode]

Run ./.github/bin/install-zig
Downloading Zig release archive...
curl: (22) The requested URL returned error: ...

We could update .github/bin/install-zig to show the requested url:

echo "Downloading Zig release archive (${url})..." >&2

[ee7]

Let's see if macos-13 works.

[ee7]

CI failed with macos-14 because that changed the macOS job from x86_64 to arm64, and it looks like the install-zig script doesn't support arm64 macOS properly. We can consider fixing that later, or moving to a different zig installation method.

But for now, I've merged this with macos-13, which is still x86_64.

We can still consider adding macos-14 to the matrix later. I'd lean towards running on both arm64 macOS and x86_64 macOS, rather than just one of them.