Workaround for MacOS+OpenMPI4 (#1375)

Use TCP BTL for MPI in GitHub Actions to fix sporadic SEGFAULT on MacOS.

See open-mpi/ompi#6518

.github/workflows/basic.yml

@@ -88,6 +88,9 @@ jobs:
         CXX:        ${{ matrix.config.cxx }}
         # We set PYTHONPATH instead of installing arbor to avoid distribution/OS specific behaviour.
         PYTHONPATH: ${{ github.workspace }}/build/python
+        # This is a workaround for the unfortunate interaction of MacOS and OpenMPI 4
+        # See https://github.com/open-mpi/ompi/issues/6518
+        OMPI_MCA_btl: "self,tcp"
     steps:
       - name: Set up cmake
         uses: jwlawson/actions-setup-cmake@v1.7