CI: add a workaround for macOS builders on GHA

The GHA agents have installed capstone via brew.  Unfortunately, the
pkg-config data for the bottle provides an include path with an extra
component which we cannot reliably strip in the build system.  The
result is that `capstone/capstone.h` is not found.  Explicitly inject an
additional header search path from the command line to resolve the
header properly.

.github/workflows/build.yml

@@ -62,8 +62,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { cflags: "", cxxflags: "" }
-          - { cflags: "-fmodules", cxxflags: "-fmodules" }
+          - { cflags: "", cxxflags: "-I/usr/local/Cellar/capstone/4.0.2/include" }
+          - { cflags: "-fmodules", cxxflags: "-fmodules -I/usr/local/Cellar/capstone/4.0.2/include" }
 
     steps:
       - uses: seanmiddleditch/gha-setup-ninja@master
@@ -96,7 +96,7 @@ jobs:
         with:
           path: ${{ github.workspace }}/bloaty
       - name: configure
-        run: cmake -B build/bloaty -D CMAKE_BUILD_TYPE=Debug -D CMAKE_C_FLAGS=${{ matrix.cflags }} -D CMAKE_CXX_FLAGS=${{ matrix.cxxflags }} -G Ninja -S ${{ github.workspace }}/bloaty -D FILECHECK_EXECUTABLE=${{ github.workspace }}/build/llvm-project/bin/FileCheck -D YAML2OBJ_EXECUTABLE=${{ github.workspace }}/build/llvm-project/bin/yaml2obj -D LIT_EXECUTABLE=${{ github.workspace }}/llvm-project/llvm/utils/lit/lit.py
+        run: cmake -B build/bloaty -D CMAKE_BUILD_TYPE=Debug -D CMAKE_C_FLAGS="${{ matrix.cflags }}" -D CMAKE_CXX_FLAGS="${{ matrix.cxxflags }}" -G Ninja -S ${{ github.workspace }}/bloaty -D FILECHECK_EXECUTABLE=${{ github.workspace }}/build/llvm-project/bin/FileCheck -D YAML2OBJ_EXECUTABLE=${{ github.workspace }}/build/llvm-project/bin/yaml2obj -D LIT_EXECUTABLE=${{ github.workspace }}/llvm-project/llvm/utils/lit/lit.py
       - name: build
         run: cmake --build build/bloaty --config Debug
       - name: test