Use OpenMPI instead of MPICH on Ubuntu in CI

The mpi4py/setup-mpi action applies a hotfix for a broken MPICH package
on Ubuntu 24.04 (Launchpad #2072338) by downloading a patch via curl.
This curl step intermittently fails with a connection reset, breaking
otherwise-unrelated CI runs. Switching to OpenMPI avoids the hotfix path
entirely and is consistent with the openmpi packages already installed
by the Linux system dependencies step.

Author: apdavison

.github/workflows/full-test.yml

@@ -21,6 +21,11 @@ jobs:
     steps:
     - uses: actions/checkout@v6
     - uses: mpi4py/setup-mpi@v1
+      if: startsWith(matrix.os, 'ubuntu')
+      with:
+        mpi: openmpi
+    - uses: mpi4py/setup-mpi@v1
+      if: startsWith(matrix.os, 'windows')
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with: