.github: Attempt to fix and increase robustness of macOS action

.github: Attempt to fix and increase robustness of macOS action

Homebrew has added LLVM 14 and made that the default version, but GitHub
continues to install LLVM 13 for now, so it ends up only accessible via
the versioned name and not the unversioned one. We also add an explicit
installation of llvm@13 so that, if GitHub updates the image to using
LLVM 14, the action continues to work, albeit slightly more slowly. This
also ensures the compiler label remains correct rather than outdated, as
has occurred in the past, and that we don't get new versions of LLVM
before we're ready for them, which is especially relevant for stable
branches. This all mirrors how the Ubuntu jobs are configured.