[nix] use extend mechanism also for non-default llvm versions

nix/default.nix

@@ -10,7 +10,6 @@ let frama_clang_build =
     llvm_version,
     llvm?pkgs.${"llvm_"+llvm_version},
     llvm_package?pkgs.${"llvmPackages_"+llvm_version} } :
-
 (plugins.helpers.simple_plugin
    { inherit pkgs stdenv src opam2nix ocaml_version plugins;
      name = "frama-clang-on-llvm-" + llvm_version;
@@ -34,5 +33,8 @@ in
 let new_pkgs = import newer_nix {};
 in
 (frama_clang_build { llvm_version="9"; })
-// { on-llvm10 = (frama_clang_build { llvm_version="10"; });}
-// { on-llvm11 = (frama_clang_build { pkgs = new_pkgs; llvm_version="11";});}
+  .extend(
+    self: super:
+    { on-llvm10 = (frama_clang_build { llvm_version="10"; });
+      on-llvm11 = (frama_clang_build { pkgs = new_pkgs; llvm_version="11";});
+    })