Update nixpkgs snapshot to one containing libtensorflow v2.4.2

The previous snapshot contained a libtensorflow version that did not
match the third_party/tensorflow submodule version.

Author: Bart Schuurmans

stack.yaml

@@ -27,6 +27,6 @@ extra-include-dirs:
 
 nix:
     enable: false
-    # release-19.09 with libtensorflow 1.14.0
-    path: ["nixpkgs=https://github.com/NixOS/nixpkgs/archive/8731aaaf8b30888bc24994096db830993090d7c4.tar.gz"]
+    # nixos-21.11 with libtensorflow 2.4.2
+    path: ["nixpkgs=https://github.com/NixOS/nixpkgs/archive/ce635e9dca8f7e2bfab19a3667d7e697c019c68b.tar.gz"]
     packages: [snappy, zlib, protobuf, libtensorflow]