chore(nix): Set npins dir with direnv

.envrc

@@ -1 +1,12 @@
+#!/usr/bin/env bash
+# the shebang is ignored, but nice for editors
+watch_file nix/sources.json
+
+# Load .env file if it exists
+dotenv_if_exists
+
+# Set npins dir
+export NPINS_DIRECTORY="nix"
+
+# Activate development shell
 use nix

shell.nix

@@ -8,6 +8,4 @@ pkgs.mkShellNoCC {
     colmena
     nixfmt-rfc-style
   ];
-
-  NPINS_DIRECTORY = "nix";
 }