[ghactions] Link to brew's libomp

Explicitly install and link libomp from homebrew on macOS,
removing the KMP_DUPLICATE_LIB_OK workaround.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Wentzell

.github/workflows/build.yml

@@ -108,13 +108,14 @@ jobs:
       if: ${{ contains(matrix.os, 'macos') }}
       run: |
         brew update
-        brew install ccache gcc llvm boost fftw hdf5 open-mpi openblas
+        brew install ccache gcc llvm libomp boost fftw hdf5 open-mpi openblas
         pip install mako numpy scipy mpi4py cvxpy
         pip install -r requirements.txt
         echo "PATH=$(brew --prefix llvm)/bin:$(brew --prefix gcc)/bin:$PATH" >> $GITHUB_ENV
         echo "PYTHONPATH=$(brew --prefix llvm)/lib/python3.13/site-packages" >> $GITHUB_ENV
         echo "SDKROOT=$(xcrun --show-sdk-path)" >> $GITHUB_ENV
-        echo "LDFLAGS=-L$(brew --prefix llvm)/lib/c++ -L/opt/homebrew/opt/llvm/lib/unwind -lunwind" >> $GITHUB_ENV
+        echo "CPPFLAGS=-I$(brew --prefix libomp)/include $CPPFLAGS" >> $GITHUB_ENV
+        echo "LDFLAGS=-L$(brew --prefix libomp)/lib -L$(brew --prefix llvm)/lib/c++ -L/opt/homebrew/opt/llvm/lib/unwind -lunwind" >> $GITHUB_ENV
 
     - name: Add clang CXXFLAGS
       if: ${{ startsWith(matrix.cxx, 'clang') }}
@@ -154,7 +155,6 @@ jobs:
       env:
         OMP_NUM_THREADS: "1"
         OPENBLAS_NUM_THREADS: "1"
-        KMP_DUPLICATE_LIB_OK: "TRUE"
       run: |
         source $HOME/install/share/triqs/triqsvars.sh
         cd build