serac build mixes OpenMPI and MPICH when both are installed

Nobody has claimed this yet.

I have MPICH installed into ```/usr/local``` and OpenMPI installed into ```/usr/local/mpi/openmpi```.

When OpenMPI is selected through cmake options serac binaries are linked with both MPI libraries:
```
$ ldd -a work/stage/usr/local/bin/serac | more
/disk-samsung/freebsd-ports/science/serac/work/stage/usr/local/bin/serac:
libserac_physics.so => not found (0x200000)
libserac_coefficients.so => not found (0x200000)
libserac_physics_integrators.so => not found (0x200000)
libserac_physics_materials.so => not found (0x200000)
libserac_numerics.so => not found (0x200000)
libserac_boundary_conditions.so => not found (0x200000)
libserac_state.so => not found (0x200000)
libserac_mesh.so => not found (0x200000)
libserac_infrastructure.so => not found (0x200000)
libaxom.so => /usr/local/lib/libaxom.so (0x200000)
libconduit_relay_mpi_io.so => /usr/local/lib/libconduit_relay_mpi_io.so (0x200000)
libconduit_relay.so => /usr/local/lib/libconduit_relay.so (0x200000)
librt.so.1 => /usr/lib/librt.so.1 (0x200000)
libhdf5.so.200 => /usr/local/lib/libhdf5.so.200 (0x200000)
libsiloh5.so.0 => /usr/local/lib/libsiloh5.so.0 (0x200000)
libm.so.5 => /lib/libm.so.5 (0x200000)
libdl.so.1 => /usr/lib/libdl.so.1 (0x200000)
libconduit_blueprint_mpi.so => /usr/local/lib/libconduit_blueprint_mpi.so (0x200000)
libconduit_blueprint.so => /usr/local/lib/libconduit_blueprint.so (0x200000)
libconduit_relay_mpi.so => /usr/local/lib/libconduit_relay_mpi.so (0x200000)
libconduit.so => /usr/local/lib/libconduit.so (0x200000)
liblua-5.4.so => /usr/local/lib/liblua-5.4.so (0x200000)
libmfem.so.4.4.0 => /usr/local/lib/libmfem.so.4.4.0 (0x200000)
libHYPRE.so => /usr/local/lib/libHYPRE.so (0x200000)
libmpicxx.so.12 => /usr/local/lib/libmpicxx.so.12 (0x200000)
libmpi.so.12 => /usr/local/lib/libmpi.so.12 (0x200000)
libmpi.so.40 => /usr/local/mpi/openmpi/lib/libmpi.so.40 (0x200000)
libmpi_cxx.so.40 => /usr/local/mpi/openmpi/lib/libmpi_cxx.so.40 (0x200000)
libc++.so.1 => /usr/lib/libc++.so.1 (0x200000)
libcxxrt.so.1 => /lib/libcxxrt.so.1 (0x200000)
libgcc_s.so.1 => /usr/local/lib/gcc11/libgcc_s.so.1 (0x200000)
libthr.so.3 => /lib/libthr.so.3 (0x200000)
libc.so.7 => /lib/libc.so.7 (0x200000)
```

cmake arguments:
-DAXOM_DIR=/usr/local -DCONDUIT_DIR=/usr/local -DMFEM_DIR=/usr/local -DENABLE_MPI=ON -DMPI_C_COMPILER=/usr/local/mpi/openmpi/bin/mpicc -DMPI_CXX_COMPILER=/usr/local/mpi/openmpi/bin/mpicxx -DCMAKE_C_COMPILER:STRING="cc" -DCMAKE_CXX_COMPILER:STRING="c++" -DCMAKE_C_FLAGS:STRING="-O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing " -DCMAKE_C_FLAGS_DEBUG:STRING="-O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing " -DCMAKE_C_FLAGS_RELEASE:STRING="-O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing " -DCMAKE_CXX_FLAGS:STRING="-O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing -fno-omit-frame-pointer " -DCMAKE_CXX_FLAGS_DEBUG:STRING="-O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing -fno-omit-frame-pointer " -DCMAKE_CXX_FLAGS_RELEASE:STRING="-O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -fno-strict-aliasing -fno-omit-frame-pointer " -DCMAKE_EXE_LINKER_FLAGS:STRING=" -fstack-protector-strong " -DCMAKE_MODULE_LINKER_FLAGS:STRING=" -fstack-protector-strong " -DCMAKE_SHARED_LINKER_FLAGS:STRING=" -fstack-protector-strong " -DCMAKE_INSTALL_PREFIX:PATH="/usr/local" -DCMAKE_BUILD_TYPE:STRING="Release" -DTHREADS_HAVE_PTHREAD_ARG:BOOL=YES -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=YES -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DBUILD_SHARED_LIBS:BOOL=ON -DENABLE_TESTS:BOOL=OFF -GNinja -DMPIEXEC_EXECUTABLE:FILEPATH="/usr/local/mpi/openmpi/bin/mpiexec" -DMPI_HOME:PATH="/usr/local/mpi/openmpi"

axom, for example, doesn't have such problem.

Reproduce the issue with the posted CMake arguments while both MPICH and OpenMPI are installed, then inspect the generated link dependencies for serac and its libraries. Done means selecting /usr/local/mpi/openmpi results in binaries linked only against the OpenMPI libraries, without MPICH dependencies.

Written by the indexing model from the issue text.