fix(ci): Use homebrew installed llvm to build clp dependencies in macOS GitHub action runner (fixes #1040).

.github/workflows/clp-core-build-macos.yaml

@@ -74,6 +74,21 @@ jobs:
       - run: "./tools/scripts/deps-download/init.sh"
         shell: "bash"
 
+      # NOTE: We set up the environment variables for LLVM so that the dependencies and CLP core
+      # are built with clang/clang++ from homebrew installed LLVM. Need to unset for clang-tidy.
+      # See https://github.com/y-scope/clp/issues/1080 for details and exploration of solutions.
+      - name: "Set up environment variables for LLVM"
+        shell: "bash"
+        run: |
+          LLVM_PREFIX=$(brew --prefix llvm@16)
+          {
+            echo "LLVM_PREFIX=$LLVM_PREFIX"
+            echo "CC=$LLVM_PREFIX/bin/clang"
+            echo "CXX=$LLVM_PREFIX/bin/clang++"
+            echo "AR=$LLVM_PREFIX/bin/llvm-ar"
+            echo "RANLIB=$LLVM_PREFIX/bin/llvm-ranlib"
+          } >> "$GITHUB_ENV"
+
       - run: "CLP_CORE_MAX_PARALLELISM_PER_BUILD_TASK=$(getconf _NPROCESSORS_ONLN) task deps:core"
         shell: "bash"
 
@@ -105,6 +120,17 @@ jobs:
           # violations.
           key: "main-branch-${{matrix.os}}-lint:check-cpp-static-full"
 
+      - name: "Unset LLVM environment variables"
+        shell: "bash"
+        run: >-
+          {
+            echo "LLVM_PREFIX="
+            echo "CC="
+            echo "CXX="
+            echo "AR="
+            echo "RANLIB="
+          } >> "$GITHUB_ENV"
+
       # TODO: When enough files are passing clang-tidy, switch to a full pass on schedule only.
       # - run: >-
       #     task lint:check-cpp-${{(github.event_name == 'schedule') && 'full' || 'diff'}}