Fix build-centos-7: expand CC/CXX paths via github.workspace
node-gyp >=12 added GetCompilerPredefines() which calls the compiler via
Python subprocess.run (no shell), so \${PWD} in CC/CXX is never expanded.
Replace with \${{ github.workspace }} which GitHub Actions expands before the
step runs, giving a static absolute path.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>