Use explicit order in overlays

m/common/main.nix

@@ -9,11 +9,12 @@
     ./slurm.nix
     ./ssh.nix
     ./users.nix
-
-    ../../pkgs/mpi.nix
   ];
 
-  nixpkgs.overlays = [ bscpkgs.bscOverlay ];
+  nixpkgs.overlays = [
+    bscpkgs.bscOverlay
+    (import ../../pkgs/mpi.nix)
+  ];
 
   nix.nixPath = [
     "nixpkgs=${nixpkgs}"

pkgs/mpi.nix

@@ -1,6 +1,5 @@
-let
-  overlay = final: prev:
-  {
+final: prev:
+{
   bsc = prev.bsc.extend (bsc: {
     mpi2 = bsc.mpich;
     mpich = with final; prev.mpich.overrideAttrs (old: {
@@ -22,10 +21,5 @@ let
         "FCFLAGS=-fallow-argument-mismatch"
       ];
     });
-    };
-  };
-in
-
-{
-  nixpkgs.overlays = [ overlay ];
+  });
 }