[ci] Use proper shared object extname on macOS

Author: feisuzhu

.github/workflows/scripts/aot-demo.sh

@@ -173,7 +173,13 @@ function check-c-api-export-symbols {
     python3 .github/workflows/scripts/build.py wheel --permissive --write-env=/tmp/ti-aot-env.sh
     . /tmp/ti-aot-env.sh
 
-    LIBTAICHI_C_API=$TAICHI_C_API_INSTALL_DIR/lib/libtaichi_c_api.so
+    UNAME=$(uname)
+    if [ "$UNAME" == "Linux" ]; then
+        EXT="so"
+    elif [ "$UNAME" == "Darwin" ]; then
+        EXT="dylib"
+    fi
+    LIBTAICHI_C_API=$TAICHI_C_API_INSTALL_DIR/lib/libtaichi_c_api.$EXT
 
     # T: global functions
     # B: global variables (uninitialized)