Update MSYS2 job in GitHub Actions workflow

- Remove unavailable mingw-w64-x86_64-openmpi package
- Disable MPI in CMake configuration

This change allows the MSYS2 Mingw-w64 build to proceed without
errors, though it disables MPI support. Future work may be needed
to re-enable MPI using an alternative package or build method.

relevant to #494

.github/workflows/build.yaml

@@ -115,7 +115,6 @@ jobs:
             mingw-w64-x86_64-gcc-fortran
             mingw-w64-x86_64-cmake
             mingw-w64-x86_64-openblas
-            mingw-w64-x86_64-openmpi
             mingw-w64-x86_64-parmetis
             make
       
@@ -131,7 +130,7 @@ jobs:
             -DWITH_Zoltan=OFF \
             -DWITH_Mumps=OFF \
             -DCREATE_PKGCONFIG_FILE=ON \
-            -DWITH_MPI=ON
+            -DWITH_MPI=OFF
       
       - name: Build
         run: |