deploy: Do not fail with broken local config

shell.nix

@@ -23,6 +23,10 @@ let
 
     deploy = ''
       set -e
+      # If a deployment to localhost introduces an evaluation error, future
+      # deployments will fail since the overlay compat can’t parse the
+      # configuration. Overlays aren’t needed to build the krops deploy script.
+      export NIX_PATH="$(sed 's/:nixpkgs-overlays=[^:]*//' <<< "$NIX_PATH")"
       $(nix-build --no-out-link deploy.nix -A "$1")
     '';