CI: Avoid unsupported UCX transport on GitHub runners

Force NVHPC's HPC-X Open MPI to use the ob1 PML because UCX cannot reliably initialise the Azure MANA device used by GitHub-hosted runners.

Author: mlange05

.github/workflows/build.yml

@@ -223,6 +223,11 @@ jobs:
         # Add mpirun to path for testing
         [ -z ${MPI_HOME+x} ] || echo "${MPI_HOME}/bin" >> $GITHUB_PATH
 
+        # HPC-X UCX cannot initialise the Azure MANA device on some GitHub runners.
+        if [[ "${{ matrix.compiler }}" == nvhpc-* ]]; then
+          echo "OMPI_MCA_pml=ob1" >> $GITHUB_ENV
+        fi
+
     - name: Install HDF5
       if: contains( matrix.compiler, 'intel' ) || contains(matrix.compiler, 'nvhpc') || contains(matrix.compiler, 'flang')
       run: |