The macos-15 runner is arm64. Previously, cp38 was the first Python in
the cibuildwheel matrix, and because the cp38 macOS installer is
x86_64-only, cibuildwheel ran the build under \`arch -x86_64\`, which
also caused CMake to build libcapnp as x86_64. Subsequent Python
versions then reused that cached x86_64 libcapnp.
After dropping cp38, cp39 is built first and CMake auto-detects the
host arch (arm64), producing an arm64 libkj/libcapnp. The x86_64 wheel
then silently links via \`-undefined dynamic_lookup\` (the linker just
ignores arm64 archive members) and fails at dlopen time with
"symbol not found in flat namespace ... AsyncIoStream::getsockopt".
Set CMAKE_OSX_ARCHITECTURES to the matrix arch on macOS so libcapnp is
always built for the wheel's target architecture.
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