Replace mpi inside bsc attribute

pkgs/mpi.nix

@@ -1,6 +1,8 @@
 let
   overlay = final: prev:
   {
+    bsc = prev.bsc.extend (bsc: {
+      mpi2 = bsc.mpich;
       mpich = with final; prev.mpich.overrideAttrs (old: {
         buildInput = old.buildInputs ++ [
           libfabric
@@ -21,6 +23,7 @@ let
         ];
       });
     };
+  };
 in
 
 {