Add MacOS to CI

There should be some CI for MacOS. However this is not trivial due to the current use of the `epics-containers/epics-base-linux-developer` container. This container provides our Ubuntu runs with a pre-installed EPICS setup, which saves a lot of setup during the tests.

However, we cannot use this same container on MacOS, and cannot simply just add the "macos-latest" to the `matrix.os` list as it'll inherit use of this EPICS base container.

We either need to create a split Linux and MacOS test jobs, or use `epicscorelibs` to get an EPICS installation. Ideally we do not want to always use `epicscorelibs`, as we want to test `catools`'s ability to load EPICS from a base installation.