geant4.data: refactor to use callPackage

This should enable some overriding capability.

pkgs/development/libraries/physics/geant4/datasets.nix

···
-{ lib, stdenv, fetchurl, geant_version }:
+{ lib, stdenv, fetchurl, geant4 }:
 
 let
   mkDataset = { pname, version, sha256, envvar }:
     stdenv.mkDerivation {
       inherit pname version;
-      inherit geant_version;
+      geant_version = geant4.version;
 
       src = fetchurl {
         url = "https://cern.ch/geant4-data/datasets/${pname}.${version}.tar.gz";
···
       dontBuild = true;
       dontConfigure = true;
 
-      datadir = "${placeholder "out"}/share/Geant4-${geant_version}/data/${pname}${version}";
+      datadir = "${placeholder "out"}/share/Geant4-${geant4.version}/data/${pname}${version}";
       installPhase = ''
         mkdir -p $datadir
         mv ./* $datadir

pkgs/development/libraries/physics/geant4/default.nix

···
   setupHook = ./geant4-hook.sh;
 
   passthru = {
-    data = import ./datasets.nix {
-          inherit lib stdenv fetchurl;
-          geant_version = version;
-      };
+    data = callPackage ./datasets.nix {};
 
     tests = callPackage ./tests.nix {};