Fix build_apple_frameworks.sh buck resolution

Author: shoumikhin

build/build_apple_frameworks.sh

@@ -206,9 +206,9 @@ echo "Exporting headers"
 mkdir -p "$HEADERS_PATH"
 
 BUCK2=$("$PYTHON" "$SOURCE_ROOT_DIR/tools/cmake/resolve_buck.py" --cache_dir="$SOURCE_ROOT_DIR/buck2-bin")
-if [[ -z "$BUCK2" ]]; then
-  echo "Could not find buck2 executable in any buck2-bin directory under $SOURCE_ROOT_DIR"
-  BUCK2=$(which buck2)
+if [[ ! -x "$BUCK2" ]]; then
+  echo "Buck2 not found at '$BUCK2'"
+  exit 1
 fi
 
 check_command "$BUCK2"