chore(3p/nix): Use packages from depot tree

third_party/default.nix

@@ -117,6 +117,30 @@ let
       xz
       zlib
       zstd;
+
+    # Required by //third_party/nix
+    inherit (nixpkgs)
+      autoconf-archive
+      autoreconfHook
+      aws-sdk-cpp
+      bison
+      boehmgc
+      boost # urgh
+      brotli
+      busybox
+      curl
+      docbook5
+      docbook_xsl_ns
+      editline
+      flex
+      libseccomp
+      libsodium
+      libxml2
+      libxslt
+      mercurial
+      perl
+      perlPackages
+      utillinuxMinimal;
   };
 
 in exposed // {

third_party/nix/release.nix

@@ -1,5 +1,5 @@
 { nix ? builtins.fetchGit ./.
-, nixpkgs ? builtins.fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.03.tar.gz
+, nixpkgs ? (import <nixpkgs> {}).third_party
 , officialRelease ? false
 , systems ? [ "x86_64-linux" "i686-linux" "x86_64-darwin" "aarch64-linux" ]
 }:

third_party/nix/shell.nix

@@ -1,10 +1,9 @@
-{ useClang ? false }:
+let pkgs = (import <nixpkgs> {}).third_party;
-
+in with pkgs;
-with import (builtins.fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.03.tar.gz) {};
 
 with import ./release-common.nix { inherit pkgs; };
 
-(if useClang then clangStdenv else stdenv).mkDerivation {
+clangStdenv.mkDerivation {
   name = "nix";
 
   buildInputs = buildDeps ++ propagatedDeps ++ tarballDeps ++ perlDeps;