Pin to a specific version

nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "url": "https://github.com/NixOS/nixpkgs",
+  "rev": "7e1f60dfbba67b975d1a77d710a6f1437fd9709c",
+  "date": "2020-02-01T09:20:37-05:00",
+  "sha256": "0vk55459iljr5dzwnr5661l44b0wdc15952lk2rjcmxr1620yr5v",
+  "fetchSubmodules": false
+}

release.nix

@@ -1,4 +1,13 @@
-{ pkgs ? import <nixpkgs> {} }:
+let
+  pin = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
+  pinnedPkgs = with pin; import (builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
+    inherit sha256;
+  }) {};
+
+in
+
+{ pkgs ? pinnedPkgs }:
 
 let
   haskellPackages = pkgs.haskellPackages;