[nix] pin channel to 20.09

There are issues in the 21.05 channel related to python 2.7 deprecations

Author: sjkelly

dev/nix/shell.nix

@@ -1,5 +1,16 @@
 with import <nixpkgs> {};
 
+# Pin the nixpkgs channel
+with import (builtins.fetchTarball {
+  # Descriptive name to make the store path easier to identify
+  name = "nixos-2020-09";
+  # Commit hash
+  url = "https://github.com/NixOS/nixpkgs/archive/20.09.tar.gz";
+  # Hash obtained using `nix-prefetch-url --unpack <url>`
+  sha256 = "1wg61h4gndm3vcprdcg7rc4s1v3jkm5xd7lw8r2f67w502y94gcy";
+}) {};
+
+
 let
   python_packages = p: with p; [
     pip