fix ssh-agent issue and added new file where pinned nixpkgs are specified

shell.nix

@@ -1,10 +1,5 @@
 let
-  # Pin nixpkgs
-  nixpkgs = builtins.fetchGit {
-    url = "https://github.com/nixos/nixpkgs-channels";
-    ref = "nixos-18.09";
-    rev = "001b34abcb4d7f5cade707f7fd74fa27cbabb80b";
-  };
+  nixpkgs = (import ./nixpkgs-pinned.nix).nixpkgs;
 in
 with import nixpkgs { };
 
@@ -15,6 +10,11 @@ stdenv.mkDerivation rec {
 
   shellHook = ''
     export NIX_PATH="nixpkgs=${nixpkgs}:."
+    # ssh-agent and nixops don't play well together (see
+    # https://github.com/NixOS/nixops/issues/256). I'm getting `Received disconnect
+    # from 10.1.1.200 port 22:2: Too many authentication failures` if I have a few
+    # keys already added to my ssh-agent.
+    export SSH_AUTH_SOCK=""
     figlet "nix-bitcoin"
     ./secrets/generate_secrets.sh
   '';