shell.nix: add missing gnupg

This should fix secretstore on NixOS

Change-Id: Id86b0e920bef82f08a67a84e59d37d6f8737d83f
diff --git a/shell.nix b/shell.nix
index e52c741..080aca0 100644
--- a/shell.nix
+++ b/shell.nix
@@ -36,6 +36,7 @@
     tmate
     git
     which
+    gnupg
     nettools
   ];
   multiPkgs = pkgs: [