Pin haskell.nix

This avoids needless build churn and potentially destabilizing changes from
upstream (though the latter does not seem likely since upstream changes should
not affect packages in our resolver, only newer resolvers).

nix/default.nix

 { pkgs ? import <nixpkgs> {} }:
 
 let
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) { inherit pkgs; };
+  # Pin a particular version of haskell.nix.  The particular version isn't
+  # special.  It's just recent at the time this expression was written and it
+  # is known to work with PaymentServer.  It could be bumped if necessary but
+  # this would probably only happen as a result of bumping the resolver in
+  # stack.yaml.
+  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/0cb32e695d7014908fb01fd7e3d225ea33dbdc98.tar.gz) { inherit pkgs; };
 
   pkgSet = haskell.mkStackPkgSet {
     stack-pkgs = import ./pkgs.nix;