Define the `WASP_SERVER_URL` env var

[infomiho]

Before we did the OAuth redirect like this:

[server] ---> [Google] ---> [client] --> (client sends the Google code to server)

We need the serverUrl variable for the new OAuth setup since we are doing the redirect like this:

[server] ---> [Google] --> [server (gets the code directly from the redirect)] ---> [client]