Try with mpi=1 and include macOS 14

.github/workflows/mcstas-basictest.yml

@@ -241,7 +241,7 @@ jobs:
 
     - name: Launch NCrystal test instrument (mpi)
       id: ncrystal-test-mpi
-      if: runner.os == 'Linux' || matrix.os == 'macos-15'  # Linux or macOS-15 only
+      if: runner.os == 'Linux' || runner.os == 'macOS'  # Linux or macOS only
       # Status: Works on Unixes ("ncrystal missing for Windows")
       run: |
            set -e
@@ -257,7 +257,11 @@ jobs:
            clang --version
            mpirun --version
            ncrystal-config --version
+           if [ "x$(uname)" == "xLinux" ]; then
            export NUM_MPI=2
+           else
+               export NUM_MPI=1
+           fi
            ../install_mcstas/bin/${MCRUN_EXECUTABLE} --mpi=${NUM_MPI} NCrystal_example.instr sample_cfg="Y2O3_sg206_Yttrium_Oxide.ncmat\;density=0.6x"
 
     - name: Launch NeXus test instrument