mpiCheckPhaseHook: change HYDRA_IFACE to lo0 on Darwin platform (#398389)

pkgs/by-name/mp/mpiCheckPhaseHook/mpi-check-hook.sh

···
       ;;
     MPICH)
       # Fix to make mpich run in a sandbox
-      export HYDRA_IFACE=lo
+      export HYDRA_IFACE="@iface@"
       # Disable sysfs cpu topology directory discovery.
       export HWLOC_XMLFILE="@topology@"
       ;;

pkgs/by-name/mp/mpiCheckPhaseHook/package.nix

···
-{ callPackage, makeSetupHook }:
+{
+  callPackage,
+  makeSetupHook,
+  stdenv,
+}:
 
 makeSetupHook {
   name = "mpi-checkPhase-hook";
 
   substitutions = {
+    iface = if stdenv.hostPlatform.isDarwin then "lo0" else "lo";
     topology = ./topology.xml;
   };
 } ./mpi-check-hook.sh