build: fix nix shell (#9288)

* build: fix deprecated stdenv.lib in nix shell

* build: fix libclang_path in nix shell

* build: update rust toolchain in nix shell

substrate/shell.nix

 let
   mozillaOverlay =
     import (builtins.fetchGit {
-      url = "https://github.com/mozilla/nixpkgs-mozilla.git";
-      rev = "57c8084c7ef41366993909c20491e359bbb90f54";
+      # TODO: revert to upstream after https://github.com/mozilla/nixpkgs-mozilla/pull/250
+      url = "https://github.com/andresilva/nixpkgs-mozilla.git";
+      rev = "7626aca57c20f3f6ee28cce8657147d9b358ea18";
     });
   nixpkgs = import <nixpkgs> { overlays = [ mozillaOverlay ]; };
-  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2021-03-01"; channel = "nightly"; }).rust.override {
+  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2021-07-06"; channel = "nightly"; }).rust.override {
     targets = [ "wasm32-unknown-unknown" ];
   });
 in
@@ -14,11 +15,11 @@ with nixpkgs; pkgs.mkShell {
     clang
     pkg-config
     rust-nightly
-  ] ++ stdenv.lib.optionals stdenv.isDarwin [
+  ] ++ lib.optionals stdenv.isDarwin [
     darwin.apple_sdk.frameworks.Security
   ];
 
-  LIBCLANG_PATH = "${llvmPackages.libclang}/lib";
+  LIBCLANG_PATH = "${llvmPackages.libclang.lib}/lib";
   PROTOC = "${protobuf}/bin/protoc";
   ROCKSDB_LIB_DIR = "${rocksdb}/lib";
 }