[CI] Explicitly set clang/clang++ as C/C++ compilers for macOS

When not setting CMAKE_{C,CXX}_COMPILER explicitly, the default
ones (cc and c++) are picked up, but are not properly setup to
find headers, which causes build failures of the Python bindings.

.github/workflows/build.yml

@@ -333,6 +333,8 @@ jobs:
       run: |
         cd ..
         cmake \
+            -DCMAKE_C_COMPILER=clang \
+            -DCMAKE_CXX_COMPILER=clang++ \
             -DCMAKE_INSTALL_PREFIX=/usr/local/ \
             -DOPENSSL_SSL_LIBRARY=$(find $(find $(brew --cellar openssl) -type d -iname "lib") -type f -iname "libssl*.dylib") \
             -DOPENSSL_CRYPTO_LIBRARY=$(find $(find $(brew --cellar openssl) -type d -iname "lib") -type f -iname "libcrypto*.dylib") \