[nix] update to new interface for plug-in derivation

nix/default.nix

 # paramaterised derivation with dependencies injected (callPackage style)
  { pkgs, stdenv, src ? ../., opam2nix,
-   ocaml_version ? "ocamlPackages_latest.ocaml", plugins ? { },
+   ocaml ? "ocamlPackages_latest.ocaml", plugins ? { },
    plugin_extend ? self: super: { }
  }:
 
@@ -13,7 +13,7 @@ let frama_clang_build =
     llvm?pkgs.${"llvm_"+llvm_version},
     llvm_package?pkgs.${"llvmPackages_"+llvm_version} } :
 (plugins.helpers.simple_plugin
-   { inherit pkgs stdenv src opam2nix ocaml_version plugins plugin_extend;
+   { inherit pkgs stdenv src opam2nix ocaml plugins plugin_extend;
      name = "frama-clang-on-llvm-" + llvm_version;
      deps = [ llvm_package.clang-unwrapped llvm pkgs.gnused ];
      opamPackages = [ "camlp5=7.14" ];