added pixi setup to the macos CI

Author: sanathkeshav

.github/workflows/build_and_test_mac.yaml

@@ -31,6 +31,9 @@ jobs:
           brew install open-mpi --build-from-source --cc=gcc-14
           brew install fftw hdf5-mpi eigen
 
+      - name: Set up pixi
+        uses: prefix-dev/[email protected]
+
       - name: Generate build directory
         run: mkdir -p ${{ env.FANS_BUILD_DIR }}