Achieve nixpkgs pinning in a different way

README.rst

@@ -7,6 +7,10 @@ Building
 --------
 
 The build system uses `Nix`_ which must be installed before anything can be built.
+Builds are tested against a particular nixpkgs revision.
+Start by exporting ``NIX_PATH`` to ensure you use the same revision::
+
+  $ export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/$(cat nixpkgs.rev).tar.gz
 
 Documentation
 ~~~~~~~~~~~~~

nixpkgs.nix

-# Pin the deployment package-set to a specific version of nixpkgs.  This is
-# NixOS 19.09 as of Oct 2 2019.  There's nothing special about it.  It's just
-# recent at the time of development.  It can be upgraded when there is value
-# in doing so.  Meanwhile, our platform doesn't shift around beneath us in
-# surprising ways as time passes.
-import (builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs-channels/archive/5d5cd70516001e79516d2ade8bcf31df208a4ef3.tar.gz";
-  sha256 = "042i081cfwdvcfp3q79219akypb53chf730wg0vwxlp21pzgns33";
-})

nixpkgs.rev

+8bf142e001b6876b021c8ee90c2c7cec385fe8e9