Make build reproducible

default.nix

@@ -1,6 +1,15 @@
-{ pkgs ? import <nixpkgs> {} }:
-
 let
+  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/afbf62d179bff8b3191316eab9890255b512bf78.tar.gz") {};
+
+  # reproducible source
+  gitignoreSrc = pkgs.fetchFromGitHub {
+    owner = "hercules-ci";
+    repo = "gitignore";
+    rev = "c4662e662462e7bf3c2a968483478a665d00e717";
+    sha256 = "sha256:1npnx0h6bd0d7ql93ka7azhj40zgjp815fw2r6smg8ch9p7mzdlx";
+  };
+  inherit (import gitignoreSrc { inherit (pkgs) lib; }) gitignoreSource;
+
   texlive = pkgs.texlive.combine {
     inherit (pkgs.texlive) scheme-small
       biber
@@ -21,7 +30,7 @@ pkgs.stdenv.mkDerivation {
     texlive
   ];
 
-  src = pkgs.nix-gitignore.gitignoreSource [] ./.;
+  src = gitignoreSource ./.;
 
   # We only build a PDF
   dontFixup = true;