cmake.yml: Downgrade to GCC 12 for macOS 12 to fix the build

Seems like GitHub Actions managed to break GCC 13 with a recent update:

Worked: macOS 12.7.4 20240514.3 + GCC 13.2.0
Broken: macOS 12.7.5 20240602.1 + GCC 13.3.0
                   ^      ^^^^^          ^

Result "not found" for test "Looking for include file sys/xattr.h"
is the earliest indicator something broke on their side during the build.

Potentially related to actions/runner-images#9997 .

.github/workflows/cmake.yml

@@ -30,7 +30,7 @@ jobs:
             clang_major_version: 18
             runs-on: ubuntu-24.04
           # macFUSE on macOS
-          - cc: gcc-13
+          - cc: gcc-12
             clang_major_version: null
             runs-on: macos-12
           - cc: clang-15