fix: reuse macos ci llvm toolchain

[Sunrisepeak]

Summary

• pin macOS CI mcpp runs to /Users/runner/.mcpp
• copy the LLVM package installed by xlings into mcpp's private registry with the .mcpp_ok marker
• avoid a second slow llvm@20.1.7 download during mcpp test on cold macOS runners

Test plan

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci-macos.yml")'
• git diff --check
• GitHub Actions CI