CI: force MacOS jobs to use Intel runners (`macos-13`)

GitHub just switched macos-latest to the ARM chips (now alisasing
`macos-14`), and it brings a bunch of problems.

- First of all, GHC's 8.8 and 8.6 don't exist there.
- ghcup and llvm are unavailable

For the time being, lets stay on the previous version of the runner.