change mpi dependency type to ("build", "run")

packages/arbor/package.py

@@ -89,7 +89,7 @@ class Arbor(CMakePackage, CudaPackage):
         depends_on("cuda@11:", when="@0.7.1:")
 
     # mpi
-    depends_on("mpi", when="+mpi")
+    depends_on("mpi", when="+mpi", type=("build", "run"))
     depends_on("py-mpi4py", when="+mpi+python", type=("build", "run"))
 
     # python (bindings)