macOS-15 only mpi x 2

.github/workflows/mcstas-basictest.yml

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