fix(nest): Align MPI variant for hdf5 dependency

@terhorst This fixes the problem where a hdf5+mpi and a nest@3.6~mpi lead to build errors by requiring a matching +mpi/~mpi variant for hdf5/nest.

packages/nest/package.py

@@ -94,7 +94,8 @@ class Nest(CMakePackage):
     depends_on('boost',             when="@2.16:+boost", type='build')
     depends_on('py-setuptools@:44.99.99',     when='@:2.15.99+python', type='build')
     depends_on('py-h5py',           when='@3.4.99:+sonata', type=('test', 'build', 'run'))
-    depends_on('hdf5+cxx',          when='@3.4.99:+sonata', type=('build', 'link', 'run'))
+    depends_on('hdf5+cxx+mpi',      when='@3.4.99:+sonata+mpi', type=('build', 'run'))
+    depends_on('hdf5+cxx~mpi',      when='@3.4.99:+sonata~mpi', type=('build', 'run'))
     depends_on('py-pandas',         when='@3.4.99:+sonata', type=('build', 'run'))
 
     depends_on('py-nose',           when='@:2.99.99+python+testsuite', type='test')