Re-introduce complete build matrix in tests

[mcw92]

Currently, some tests seem to fail randomly for MacOS in the workflow, throwing either an MPI error or running into a timeout:

=========================== short test summary info ============================
ERROR tests/test_rf_training_breaking_iid.py::test_breaking_iid[17-True-False-True-True] - FileNotFoundError: [Errno 2] No such file or directory
============= 75 passed, 1 skipped, 48 warnings, 1 error in 35.04s =============
--------------------------------------------------------------------------
MPI_ABORT was invoked on rank 3 in communicator MPI_COMM_WORLD
  Proc: [[3308,1],3]
  Errorcode: 1

NOTE: invoking MPI_ABORT causes Open MPI to kill all MPI processes.
You may or may not see output from other processes, depending on
exactly when Open MPI kills them.
--------------------------------------------------------------------------
--------------------------------------------------------------------------
prterun has exited due to process rank 3 with PID 0 on node Mac-1733739144421 calling
"abort". This may have caused other processes in the application to be
terminated by signals sent by prterun (as reported here).
--------------------------------------------------------------------------
�
Error: Process completed with exit code 1.

The Python versions of the test failing differs among runs. The error might not necessarily be on our end. As I have no idea why that is, I removed MacOS from the build matrix for now. However, it would be great if we could re-include it in a functional version again in the future.
The same applies to matrix testing of different MPI implementations.

The complete build matrix should in principle look like this. It is possible to manually exclude non-working combinations as shown in the last three lines:

jobs:
    build:
        strategy:
          matrix:
            os: ["ubuntu-latest", "macos-latest"]
            python-version: [ "3.10", "3.11", "3.12" ]
            mpi: [ "openmpi", "mpich", "intelmpi"]
            exclude:
              - os: "macos-latest"
                mpi: "intelmpi"