🐛 Fixed error when using staff access tokens

closes https://github.com/TryGhost/Team/issues/645

- we did some refactoring in [1] to turn promise chained code into
  async/await, but this removed an early `return` from the code
- therefore we'd continue on to further code, which breaks for obscure
  reasons that weren't apparent from the error
- this commit adds back a return at the end of the block where we handle
  staff API tokens to match the same functionality as before
- this is regression that landed in 4.3.0 and would break staff user
  tokens

[1]: b677927322 (diff-bc0bedcac8ec9646d0644c86a91e46f4759bc1b0c2aebac54a2b26ec474c3d15L148-L155)

core/server/services/auth/api-key/admin.js

@@ -164,6 +164,7 @@ const authenticateWithToken = async (req, res, next, {token, JWT_OPTIONS}) => {
             req.user = user;
 
             next();
+            return;
         }
 
         // store the api key on the request for later checks and logging