[nix] use default Frama-C nixpkgs version even for llvm-11

nix/default.nix

@@ -24,19 +24,9 @@ let frama_clang_build =
      '';
    });
 in
-let newer_nix =  pkgs.fetchFromGitHub {
-    owner = "nixos";
-    repo = "nixpkgs";
-    rev = "bb46a6eb7c6a0faf08263e0564c51910bfbd83c7";
-    sha256 = "15w1321wbm3vpk4qmj6d9pz3y522c0q32gkccj82c3fandb9ppj6";
-    fetchSubmodules = true;
-};
-in
-let new_pkgs = import newer_nix {};
-in
 (frama_clang_build { llvm_version="9"; })
   .extend(
     self: super:
     { on-llvm10 = (frama_clang_build { llvm_version="10"; });
-      on-llvm11 = (frama_clang_build { pkgs = new_pkgs; llvm_version="11";});
+      on-llvm11 = (frama_clang_build { llvm_version="11"; });
     })