Nix shell sync with Substrate (#100)

* nix shell config

* fix darwin

* update nightly for nix shell

shell.nix

+let
+  mozillaOverlay =
+    import (builtins.fetchGit {
+      url = "https://github.com/mozilla/nixpkgs-mozilla.git";
+      rev = "57c8084c7ef41366993909c20491e359bbb90f54";
+    });
+  nixpkgs = import <nixpkgs> { overlays = [ mozillaOverlay ]; };
+  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2020-10-05"; channel = "nightly"; }).rust.override {
+    targets = [ "wasm32-unknown-unknown" ];
+  });
+in
+with nixpkgs; pkgs.mkShell {
+  buildInputs = [
+    clang
+    cmake
+    pkg-config
+    rust-nightly
+  ] ++ stdenv.lib.optionals stdenv.isDarwin [
+    darwin.apple_sdk.frameworks.Security
+  ];
+
+  LIBCLANG_PATH = "${llvmPackages.libclang}/lib";
+  PROTOC = "${protobuf}/bin/protoc";
+  ROCKSDB_LIB_DIR = "${rocksdb}/lib";
+}