macos CI: let openmp curl fail graciously

[ben-schwen]

Follows #7318

Fixes macOS CI by selecting the OpenMP runtime from mac.r-project.org/openmp/ for the detected Apple clang version.
Avoids breakage when the pinned tarball changes (which it apparently does often for the new clang version).

The pinned OpenMP tarball still appears to exist, but GitHub Actions runners are timing out when connecting to mac.r-project.org. This change fails gracefully and falls back when the runtime cannot be fetched.

[aitap]

macrtools also hard-codes a list of files: https://github.com/coatless-mac/macrtools/blob/6d7ff513e1724741f4ebac5a3b9bf43dac905770/R/openmp.R#L367-L370

It might make sense to ask for a machine-readable list of pre-compiled runtimes, although the case may be that the OpenMP runtimes aren't "supported enough" to offer such a list.

[ben-schwen]

According to the latest R devel mails, this was a one time thing and not related to us or our checks. But for the future it still seems good that the checks won't fail once the servers are not available or blocks the GitHub runner again.