ci: add a macos builder

[garlick]

Problem: recently some changes were made to allow flux-core to compile on macos, but since we are not testing on that platform, portability issues are likely to creep back in.

We should add a CI builder for macos.

Github hosted runner for macos 14:
https://github.com/actions/runner-images/blob/main/images/macos/macos-14-Readme.md

Currently the code (including tests) should build, but unit tests will fail and sharness system tests are right out.

Edit: should have referenced #6479