[build] [mac] Fix mac build symbol export failure

[hughperkins]

Issue: #

Brief Summary

Fixes exported name check on mac build, by looking for .dylib instead of .so

copilot:summary

Walkthrough

Currently, Mac builds fail on the check exported symbols check, e.g. https://github.com/hughperkins/taichi/actions/runs/14691553460/job/41227531475?pr=2 :

/Library/Developer/CommandLineTools/usr/bin/nm: error: /Users/runner/work/taichi/taichi/_skbuild/macosx-15.0-arm64-3.9/cmake-install/python/taichi/_lib/c_api/lib/libtaichi_c_api.so: No such file or directory

The reason is that, on Mac, this file has '.dylib' extension. E.g. on my own Mac:

~/git/taichi-m1 (hp/fix-cannot-name-an-alias-template-for-trigger|…1△2) $ find . -name '*_c_api*'
./misc/generate_c_api.py
./misc/generate_c_api_docs.py
./tests/run_c_api_compat_test.py
./_skbuild/macosx-15.0-arm64-3.9/cmake-build/CMakeFiles/taichi_c_api.dir
./_skbuild/macosx-15.0-arm64-3.9/cmake-build/CMakeFiles/taichi_static_c_api_tests.dir
./_skbuild/macosx-15.0-arm64-3.9/cmake-build/CMakeFiles/taichi_c_api_tests.dir
./_skbuild/macosx-15.0-arm64-3.9/cmake-build/CMakeFiles/taichi_static_c_api.dir
./_skbuild/macosx-15.0-arm64-3.9/cmake-build/libtaichi_c_api.dylib
./_skbuild/macosx-15.0-arm64-3.9/cmake-install/python/taichi/_lib/c_api/lib/libtaichi_c_api.dylib
./_skbuild/macosx-15.0-arm64-3.9/setuptools/lib.macosx-15.0-arm64-cpython-39/taichi/_lib/c_api/lib/libtaichi_c_api.dylib
./build/taichi_static_c_api
./build/taichi_static_c_api_tests
./build/taichi_c_api_tests

This PR updates the exported name check script to look for a .dylib, when building on mac (uname == "Darwin"), which addresses this issue, and should hopefully fix the build.

copilot:walkthrough

[feisuzhu]

LGTM!

[hughperkins]

Thank you! :)