mvapich2 MPI_ROOT needed in modules (#30335)

We've found that when using mvapich2 via modules, we need to set the MPI_ROOT
environment variable.

var/spack/repos/builtin/packages/mvapich2/package.py

@@ -335,6 +335,8 @@ def setup_run_environment(self, env):
         if 'process_managers=slurm' in self.spec:
             env.set('SLURM_MPI_TYPE', 'pmi2')
 
+        env.set('MPI_ROOT', self.prefix)
+
         # Because MPI functions as a compiler, we need to treat it as one and
         # add its compiler paths to the run environment.
         self.setup_compiler_environment(env)