Rename brew clang build from clang-15 to brew-clang

We just use whichever version of clang "brew install llvm" gives us
(currently 21), so it doesn't make sense to hardcode it.  Also helps
distinguish it from the default apple clang.

Author: d-torrance

.github/workflows/test_build.yml

@@ -46,7 +46,7 @@ jobs:
           # This build tests Clang rather than AppleClang (keep)
           - build-system: cmake
             os: macos-13 # TODO: switch to macos-14 for arm64 testing
-            compiler: clang-15
+            compiler: brew-clang
             cxx: clang++
             cc: clang
         exclude: