Add AppleClang to CI tests

The version of AppleClang shipping with XCode 16 now has complete enough C++20 support to compile Flux, so we should start testing with it regularly.

Unfortunately Github are enacting a policy of only providing a single XCode major version with each MacOS image (actions/runner-images#10703), so if we want to test with e.g. XCode 17+ in future we'll have to run them on different images. But that's tomorrow's problem...

.github/workflows/macos.yml

@@ -11,25 +11,20 @@ on:
 
 jobs:
   build:
-    runs-on: macos-latest
+    runs-on: macos-15
     strategy:
       matrix:
-        compiler: [GCC-13, LLVM-Clang-16, LLVM-Clang-17, LLVM-Clang-18, LLVM-Clang-19]
+        compiler: [AppleClang, LLVM-Clang-16, LLVM-Clang-17, LLVM-Clang-18, LLVM-Clang-19]
         test_with: [Headers, Module]
         build_type: [Debug, Release]
 
         exclude:
-          - compiler: GCC-13
-        #    test_with: Module
+          - compiler: AppleClang
+            test_with: Module
           - compiler: LLVM-Clang-16
             test_with: Module
 
         include:
-        #  - compiler: GCC-13
-        #    cxx: g++-13
-        #    install: |
-        #      brew update
-        #      brew install gcc@13 ninja
           - compiler: LLVM-Clang-16
             cxx: $(brew --prefix llvm@16)/bin/clang++
             install: |