iwyu.yml: generate and use macOS mapping file

Author: firewave

.github/workflows/iwyu.yml

@@ -102,6 +102,7 @@ jobs:
       - name: Build macOS mappings
         run: |
           set -x
+
           wget https://raw.githubusercontent.com/include-what-you-use/include-what-you-use/master/mapgen/iwyu-mapgen-apple-libc.py
           python3 iwyu-mapgen-apple-libc.py $(xcrun --show-sdk-path)/usr/include > macos.imp