common/gadi/packages.yaml: add extra_attributes to openmpi spec

[harshula]

Set $CMAKE_PREFIX_PATH to workaround Gadi's openmpi directory layout. This means MPI compiler wrappers being explicitly passed to CMake are not required in SPDs, anymore.

Without this change, the upstream Spack's FMS package will not build: https://github.com/spack/spack/issues/45358

[harshula]

Testing

1) Removed MPI wrappers:

$ git -C spack-packages diff
diff --git a/packages/libaccessom2/package.py b/packages/libaccessom2/package.py
index 2ba019d..91b344b 100644
--- a/packages/libaccessom2/package.py
+++ b/packages/libaccessom2/package.py
@@ -49,10 +49,3 @@ class Libaccessom2(CMakePackage):
                         "",
                         "CMakeLists.txt"
             )
-
-    def cmake_args(self):
-        return [
-            self.define("CMAKE_C_COMPILER", self.spec["mpi"].mpicc),
-            self.define("CMAKE_CXX_COMPILER", self.spec["mpi"].mpicxx),
-            self.define("CMAKE_Fortran_COMPILER", self.spec["mpi"].mpifc),
-        ]

2) The following commands succeeded:

spack install libaccessom2 ^netcdf-c@4.7.4 ^netcdf-fortran@4.5.2 ^openmpi@4.0.2 %intel@19.0.5.281 target=x86_64 spack install libaccessom2 ^netcdf-c@4.7.4 ^netcdf-fortran@4.5.2 ^openmpi@4.1.5 %intel@2021.10.0 target=x86_64 spack install cable@git.gcc_compilation_and_warnings+mpi ^netcdf-c@4.7.4 ^netcdf-fortran@4.5.2 ^openmpi@4.1.4 %gcc@13.2.0 target=x86_64