default.nix: try new rev

Conduit testing suite does not terminate, lets try recent nixos-20.09 revision.

Author: Anton-Latukha

default.nix

@@ -91,7 +91,7 @@
 #   , nixos-20.03  # Last stable release, gets almost no updates to recipes, gets only required backports
 #   ...
 #   }
-, rev ? "daaa0e33505082716beb52efefe3064f0332b521"
+, rev ? "0cfe5377e8993052f9b0dd56d058f8008af45bd9"
 
 , pkgs ?
     if builtins.compareVersions builtins.nixVersion "2.0" < 0