Feature request: CI tests on macOS

Summary

I propose to add macOS machines to the suite of CI pipelines that test package installs.

Rationale

I often find packages don’t build on macOS (most recently paraview and its dependencies, which triggered this issue). This is a frustrating experience that can discourage new users. The main target of Spack is Linux HPC systems, but I believe users often try installing packages on their local machine as well (I do), in particular when testing Spack for the first time, and macOS is very widespread on personal machines.

Description

Builds on macOS with AppleClang work more often than LLVM clang and GCC. I propose a build matrix with all three compilers.

Additional information

For what it’s worth, GitHub Actions offers macOS virtual machines.

General information

• I have run spack --version and reported the version of Spack
• I have searched the issues of this repo and believe this is not a duplicate

Start by reviewing the existing CI pipelines that test package installs and the GitHub Actions configuration. Determine how macOS runners and AppleClang, LLVM Clang, and GCC can fit into the current matrix. Done means package-install CI runs on macOS with all three compilers and reports failures clearly.