Nixify tokens.go

- Package tokens.go with Nix
- Add monzo_ynab.{job,tokens} to shell.nix

monzo_ynab/shell.nix

@@ -1,9 +1,15 @@
-{ pkgs ? import <nixpkgs> {}, ... }:
+{
+  pkgs ? import <nixpkgs> {},
+  briefcase ? import <briefcase> {},
+  ...
+}:
 
 pkgs.mkShell {
   buildInputs = [
     pkgs.go
     pkgs.goimports
     pkgs.godef
+    briefcase.monzo_ynab.job
+    briefcase.monzo_ynab.tokens
   ];
 }