Test Miri on macOS in CI

[RalfJung]

It would be good to run the Miri tests on a macOS target in CI. With #103392, running x.py test src/tools/miri --target aarch64-apple-darwin should work on any host OS. However when we put that on CI, it fails on Windows since x.py insists that cc must be installed. So for now this is disabled in checktools.